200126260 Project Advisor

“TUBULAR RETORT WITH CONTROLLED ATMOSPHERES”

AUTHOR: JARRETT A. SMITH M. Code: 200126260

PROJECT ADVISOR: JAIRO ES COBAR M Sc, Dr Ing.

UNIVERSITY DE LOS ANDES MECHANICAL ENGINEERING DEPARTMENT BOGOTA, JUNE 2005

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TABLE OF CONTENTS

1 INTRODUCTION...... ¡ERROR! MARCADOR NO DEFINIDO. 2 OBJECTIVE...... ¡ERROR! MARCADOR NO DEFINIDO. 3 REVISION OF BIBLIOGRAPHY...... ¡ERROR! MARCADOR NO DEFINIDO. 3.1 OVERVIEW OF POWDER METALLURGY...... ¡ERROR! MARCADOR NO DEFINIDO. 3.2 OVERVIEW OF SINTERING...... ¡ERROR! MARCADOR NO DEFINIDO. 3.2.1 Physical Phenomenon - Diffusion...... ¡Error! Marcador no definido. 3.2.2 Schematic of creating a sintering cycle...... ¡Error! Marcador no definido. 3.2.2.1 Delubing stage...... ¡Error! Marcador no definido. 3.2.2.2 P reheating stage / Presintering...... ¡Error! Marcador no definido. 3.2.2.3 Hot Stage...... ¡Error! Marcador no definido. 3.2.2.4 Cooling Stage...... ¡Error! Marcador no definido. 3.2.2.5 Slow cooling stage...... ¡Error! Marcador no definido. 3.2.2.6 P urging, a used as needed stage...... ¡Error! Marcador no definido. 3.2.3 Parameters ...... ¡Error! Marcador no definido. 3.2.3.1 Temperature – Heat...... ¡Error! Marcador no definido. 3.2.3.2 Gases...... ¡Error! Marcador no definido. 3.2.3.2.1 Classification of Gases...... ¡Error! Marcador no definido. 3.2.3.2.2 Combustion (Explosion) of gases...... ¡Error! Marcador no definido. 3.2.3.2.3 Ignition (Auto-ignition) of gases...... ¡Error! Marcador no definido. 3.2.3.2.4 Gases for the atmosphere...... ¡Error! Marcador no definido. 3.2.4 Sintering Equipment...... ¡Error! Marcador no definido. 3.2.4.1 Batch Furnaces...... ¡Error! Marcador no definido. 3.2.4.2 Continuous Furnaces...... ¡Error! Marcador no definido. 3.2.4.3 Heating Units...... ¡Error! Marcador no definido. 3.2.4.4 Gas Pressure and Flow Equipment...... ¡Error! Marcador no definido. 3.2.4.5 Additional Comparisons...... ¡Error! Marcador no definido. 3.2.4.6 Thermocouples...... ¡Error! Marcador no definido. 4 METHODOLOGY...... ¡ERROR! MARCADOR NO DEFINIDO. 5 RESULTS AND ANALYSIS...... ¡ERROR! MARCADOR NO DEFINIDO.

5.1 DETAILS OF CREATING THE RETORT...... ¡ERROR! MARCADOR NO DEFINIDO. 5.1.1 Six Channel Data Acquisition...... ¡Error! Marcador no definido. 5.1.1.1 General Architecture of the System...... ¡Error! Marcador no definido. 5.1.1.2 Block Architecture of Te mperature Sensors...... ¡Error! Marcador no definido. 5.1.1.3 Implementation...... ¡Error! Marcador no definido. 5.1.2 Temperature Measurement (Thermocouple) ...... ¡Error! Marcador no definido. 5.1.3 Adequate Pressure and Flow for Mixing the Processing Gas...... ¡Error! Marcador no definido. 5.1.4 Retort construction – Shell...... ¡Error! Marcador no definido. 5.1.5 Flange of the retort...... ¡Error! Marcador no definido. 5.1.6 Seal of the Retort...... ¡Error! Marcador no definido. 5.1.7 Work load support...... ¡Error! Marcador no definido. 5.2 CHARACTERIZATION...... ¡ERROR! MARCADOR NO DEFINIDO. 5.2.1 Characterization Protocol...... ¡Error! Marcador no definido. 5.2.2 Temperature characterizations ...... ¡Error! Marcador no definido. 5.2.3 Fluid Flow Characterization...... ¡Error! Marcador no definido. 6 CONCLUSIONS...... ¡ERROR! MARCADOR NO DEFINIDO. 7 SUGGESTIONS FOR FURTHER DEVELOPMENT...... ¡ERROR! MARCADOR NO DEFINIDO. 8 BIBLIOGRAPHY...... ¡ERROR! MARCADOR NO DEFINIDO.

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Listing of Figures

Figure 1: The steps in diffusion bonding [Ref. 1]...... ¡Error! Marcador no definido. Figure 2: Diffusion of atoms to points of contact [Ref. 1]...... ¡Error! Marcador no definido. Figure 3: Generalization of activation energy...... ¡Error! Marcador no definido. Figure 4: Schematic of an Arbitrary Sintering Cycle .¡Error! Marcador no definido. Figure 5: Stress-strain curves for 304 stainless steel at vary in hydrogen concentration (given in wt%) [Ref. 8] ...... ¡Error! Marcador no definido. Figure 6: Furnace Equipment [Ref. 19] ...... ¡Error! Marcador no definido. Figure 8: Tubular Recipient Development Model...... ¡Error! Marcador no definido. Figure 9: Prototype...... ¡Error! Marcador no definido. Figure 10: Data Acquisition Card ...... ¡Error! Marcador no definido. Figure 11: General Architecture of System ...... ¡Error! Marcador no definido. Figure 12: Block Architecture of a Temperature Sensor ...... ¡Error! Marcador no definido. Figure 13: Graphical User Interface...... ¡Error! Marcador no definido. Figure 14: Format of Results Displayed with Excel ...¡Error! Marcador no definido. Figure 15: Schematic of Configuration for Forming Gas ...... ¡Error! Marcador no definido. Figure 16: A Reducing Sintering Operation Sequence ...... ¡Error! Marcador no definido. Figure 17: Material Candidates for the retort...... ¡Error! Marcador no definido. Figure 18: Operating Limits of Various Alloys in a Hydrogen Environment (Nelson Curves) [Ref. 9]...... ¡Error! Marcador no definido. Figure 19: Effect of temperature on metal loss from scaling for several carbon and alloy steels in air [Ref. 9] ...... ¡Error! Marcador no definido. Figure 20: General Comparison of the Hot-Strength Amongst Stainless Steels [Ref. 10] ...... ¡Error! Marcador no definido. Figure 21: Time-temperature curves showing effect of carbon content on carbide precipitation, which forms in the areas to the right of the various carbon-content curves.[Ref. 6]...... ¡Error! Marcador no definido. Figure 22: Family relationships for standard austenitic stainless steels ...... ¡Error! Marcador no definido. Figure 23: Cyclic Oxidation Resistance [Ref. 10] ...... ¡Error! Marcador no definido. Figure 24: Effect of Acrylonitrile Content on Permeability of Butadiene- Acrylonitrile Copolymers at 25°C...... ¡Error! Marcador no definido. Figure 25: O-ring compression force [Ref. 2]...... ¡Error! Marcador no definido. Figure 26: Linear Expansion % vs. Volume Swell %...... ¡Error! Marcador no definido.

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Listing of Tables

Table 1: Conforming Methods for Green Pieces.¡Error! Marcador no definido. Table 2: Classification of Some Common Gases¡Error! Marcador no definido. Table 3: Limits of Hydrogen by Volume Percentage...... ¡Error! Marcador no definido. Table 4: Gas Cylinder Specifications ...... ¡Error! Marcador no definido. Table 5: Types of Batch Furnaces...... ¡Error! Marcador no definido. Table 6: Types of Continuous Furnaces...... ¡Error! Marcador no definido. Table 7: Accuracy of Variable Area Flowmeters.¡Error! Marcador no definido. Table 9: Selected American Standard Pipe...... ¡Error! Marcador no definido. Table 10: Maximum service temperatures in dry air, based on scaling resistance [Ref: 6]...... ¡Error! Marcador no definido.

1 Introduction

In this modern world, industries are always searching for a manufacturing process capable of supplying large quantities of exceptionally high quality pieces at reduced costs. One of the answers of the search is powder metallurgy. With it recognized as a useful technology, efforts are continuously exerted in transferring production of parts from the competing manufacturing technologies of machining wrought steel, forging, and casting to powder metallurgy. Most of the transfer has been possible due to the associated developments in increased mechanical performance, better dimensional tolerances, and more cost efficient production of the powder metallurgy products; in other word, more knowledge and better equipment.

The great potential of powder metallurgy is achieved by following a series of steps. The first is to identify the piece fit for this manufacturing process. A piece is fit based on characteristics such as geometry, material, and size. Also, a larger quantity of pieces justifies the production of the piece based on economic feasibility. Once the preliminary requirements are met, the design of the production process follows.

One traditional route of transforming powder into a piece is by conforming a green piece and then sintering it. The design for this route will identify needs such as necessary equipment. For sintering, the equipment used are furnaces; they must be able to supply conditions which are met depending on the materials of the green piece and the end piece properties, for example density. Several types of furnaces can provide distinct sintering conditions, but they all share common conditions within that guarantee a large amount of success of sintering which are usually atmospheric composition, time dependent temperature cycles, and pressures. Only equipment capable of

1 controlling conditions produce parts as expected. Therefore, the advantages sought in the powder metallurgy manufacturing process relies heavily on the capabilities of the equipment available.

2 Objective

Powder metallurgy is a manufacturing process with a complex and vast quantity of knowledge. Therefore, tackling of all equally important aspects of powder metallurgy is a team effort. And amongst many of the key aspects is the availability of reliable and versatile equipment because without this equipment all existent knowledge and intentions to build upon that knowledge is “fiction”. Hence, this project is a consequence of a particular laboratory equipment needed for developing and building upon powder metallurgy knowledge1 with the objective of:

Design, construct, and characterize a laboratory scale retort.

3 Revision of Bibliography

Before designing the retort, a revision of bibliography is needed. It will define what possibilities and limitations the equipment should posses. With this information the parameters of the aspects that are deduced as important will be given a solution during the design stage.

1 Powder metallurgy requires a variety of equipment which are not limited just to retorts. And various designs are available for each piece o f equipment.

2 3.1 Overview of powder metallurgy

Powder metallurgy is a manufacturing process used for production components from powders followed generally by a heat treatment to produce a denser piece. The components may be small, of high complexity, of controlled properties, for high production quantities, of high melting temperatures2, of low ductility3, and with close dimensional tolerances. Depending on the component type, one of the five basic processes of powder metallurgy can be used:

• traditional conventional powder metallurgy (P/M) - compacting • metal injection molding (MIM) • powder forging (P/F) • hot isostatic pressing (HIP) • and cold isostatic pressing (CIP)

Generally, any of the five types of processes are composed of five sub- processes before obtaining the final product. A brief description of the five possible sub-processes follows. [Ref. 16]

The first is obtaining and preparing the raw materials. The powder must be obtained according to the metal. A few of the powder obtaining methods are atomization by gas and water, chemical methods, or milling of brittle materials. Then, the powder must be characterized because its property will define other sub-processes.

2 Production of metals with high melting temperatures can be expensive due to difficulty of melting and casting with other conventional manufacturing processes such as micro-casting. 3 When dealing with materials of low ductility the production technique can be limited to P/M since machining and other manufacturing processes are extremely difficult to apply.

3 Mixing comes after obtaining and preparing the raw materials. This is when the additives and/or lubricants are added or when blending and premixing of metal powders is done.

The third process is forming. Forming consists of giving the powders a functional shape and temporary properties. At this stage the piece formed is given the name of a green piece. Some of the conforming methods for green pieces are given in the table of Conforming Methods for Green Pieces.

Table 1: Conforming Methods for Green Pieces

Hot Compaction Warm Compaction Cold Compaction Isostatic Die Compacting Die Compacting Extrusion Injection Molding Isostatic Die Compacting Rolling Spray ing Injection Molding Pressureless-sintering Slip Casting Cold Forming

A green piece, when the application requires it, must then be sintered. Sintering is the fourth stage. Because the main goal of this project is to develop a recipient for use in the sintering process, sintering is left to a further, more in depth discussion which defines the design parameters.

Finally, optional operations precede achieving the final product. This stage is required only when the properties of the “as sintered” do not suffice requirement specifications. Examples of possible, additional steps are re- pressing, re-sintering, metal infiltration by lubricants, and superficial treatments. All of the steps added during this stage give added values to the powder metallurgical part.

The final products created with powder metallurgy may have geometrical complexities, functional properties, and level of quality that no other

4 manufacturing process can feasibly match; but the powder metallurgy process can only produce the high quality pieces by an adequate assessment of the previous five processes.

3.2 Overview of Sintering

A green piece, or a pre-form otherwise called, must be sintered before becoming a final product. Therefore, sintering is the process where the metal particles are given strength by the bonds between powder surfaces developed below the melting point of the mayor constituents. The rate of sintering, the rate at which bonds develop, depends on the temperature, the activation energy and diffusion coefficient for diffusion, and the original size of the particles.

Heat is the energy source for the atomic transport events, diffusion, of sintering. Thus, how the heat reaches the P/M piece must be understood. Heat transfer is possible via the following three means: radiation, conduction, and convection. Conduction and convection require the presence of a means to occur. Gas, for example, is a suitable means. In the absence of a gas, i.e. a vacuum, radiation is the only effective method of heat transfer.

Briefly summarizing, the powders of P/M must follow a sequence such as that described by the figure: The steps in diffusion bonding.

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Figure 1: The steps in diffusion bonding [Ref. 1]

Part a) of the schematic shows a section of a single powder loosely resting on another section of a single powder. The numerous sectors are grains. Densification of the powders is seen in part b), part c), and part d). In part b), a force is applied to increase contact area of the two powders. An increased amount of contact area will increase the diffusion phenomenon. Grain growth is seen in part c). Grain growth by diffusion lowers the energy levels of the particles. Finally, part d) shows a reduction of pores left by vacancy diffusion. Parts c) and d) are performed during sintering. A more in depth discussion on diffusion is found later in this document.

Sintering must also be accompanied with appropriate atmospheres. When the atmosphere of sintering is referred to, it is more than likely to explain the gaseous composition surrounding the pieces. In effect, atmosphere of sintering refers to the chemical interaction between the atmosphere and the piece and not about the heat transfer capacity of the atmosphere. Some common interactions include reduction of the piece, oxidation, and carburization. These reactions proceed when temperature, pressure, and time conditions are met.

A popular source of information is found in figures such as those that show the standard free energy of formation of metal oxides. In a figure of such type, sometimes referred to as the Ellingham Diagram, a powder

6 metallurgical practitioner can find information pertaining to whether a piece will reduce or oxidize depending upon partial pressures of the gaseous constitutes of the atmosphere at different temperatures.

Finally, sintering, as will be explained in the section: Schematic of creating a sintering cycle, is dynamic. The conditions surrounding the green pieces during sintering will change. The sub-stages of sintering are applied as necessary. This too is the explained in the same section.

3.2.1 Physical Phenomenon - Diffusion

A simple and straight forward description of diffusion is stated by Askeland [Ref. 1] as:

“Diff usion is the movement of atoms within a material. Atoms move in a predictable fashion to eliminate concentration differences and produce a homogeneous, uniform composition.”

Diffusion occurs in the retort as some of the gas penetrates the walls and as migration of atoms of the material during the sintering operation. Indisputably, diffusion must be understood for the selection of the material of the retort.

Metallic diffusion mechanisms are dominated by the vacancy diffusion model and by the interstitial diffusion model. In both of these models, for any atom to migrate there must be an empty adjacent site and the atom must have sufficient energy to break bonds with neighboring atoms and then cause some lattice distortion during the displacement.

7 A vacancy is a point defect normally due to an atom missing. Vacancy diffusion requires vacant lattice sites and is a function of the amount of vacant lattice sites available. According to William D. Callister [Ref. 4]:

“The equilibrium number of vacancies Nv for a given quantity of material depends on and increases with temperature according to:

⎛−Q ⎞ ⎜ v ⎟ ⎝ kT ⎠ N v = Ne

In this expression, N is the total number of atomic sites, Qv is the energy required for the f ormation of a vacancy, T is the absolute temperature in kelvins, and k is the gas or Boltsmann’s constant.”

It is apparent that the number of vacancies increases exponentially with an increase in temperature; being so, diffusion rate via the vacancy model increases. With self-diffusion and inter-diffusion occur, the vacancy model changes vacancies for host atoms. And in both cases, with vacancies and with host atoms, the atom that takes a step and the destination point, whether a vacancy or host atom, move in opposite directions to replace each other.

Interstitial diffusion involves the migration of an atom into an interstitial site. Thus, the migrating atom must be small enough to fit in the interstitial sites left by the host atom’s lattice. Usually the impurities that diffuse via interstitial diffusion are hydrogen, carbon, nitrogen, and oxygen due to their small sizes when compared to host atoms such as iron. Finally, interstitial diffusion, in most metal alloys, occur more rapidly because of the more numerous amounts of interstitial positions compared to vacant sites.

An example of vacancy diffusion is sintering of metal powders. The migration of atoms is quite visible from the figure: Diffusion of atoms to points of contact extracted from reference [Ref. 1]. The figure demonstrates with a before and

8 after the step like diffusion of atoms. In the first figure, the red atoms with an arrow describe the present location of the migrating atoms and the arrow points to the location they will assume. The second figure shows the atoms already in place.

Figure 2: Diffusion of atoms to points of contact [Ref. 1]

The situation describes interfacial diffusion. Interfacial diffusion first occurs at the contact area available between external surfaces of the particles. At the boundary of the surfaces the crystal structures terminate. Because the surface atoms are not bonded, they are at a higher energy state than the atoms at the interior positions giving rise to surface energy. Atoms at the surface want to reduce this energy but can not do so because they are mechanically rigid.

In a similar fashion, atoms along grain boundaries are bonded less regularly than the interior atoms (though, more bounds are obviously present along grain boundaries than external surfaces). Therefore, there is also interfacial/grain boundary energy. And at elevated temperatures grains grow to reduce the total boundary energy.

The additional amount of energy that must be added for any migration activity previously described is called activation energy. In sintering the energy is

9 provided by heat. The energy is used to squeeze through the path migration. The figure: Generalization of activation energy [Ref. 1] shows schematically the typical situation. It is usually the case that higher activation energy is required for vacancy diffusion compared to interstitial diffusion.

Figure 3: Generalization of activation energy

3.2.2 Schematic of creating a sintering cycle

A basic sintering cycle schematic is composed of typically five stages. The five stages are the following:

• Delubing (or burn-off zone) • Preheating • Hot • Cooling • Slow cooling

In each stage a different atmosphere is contained inside the retort. Each particular atmosphere has a function, a composition, and is within a temperature range (see figure: Schematic of an Arbitrary Sintering Cycle). The figure shows an arbitrary schematic of the sintering cycle with

10 proportions roughly held. Each stage is performed at within a given time and temperature or at a rate of change of these parameters. By briefly stating what these are, the requirements that must be met by the tubular recipient are deduced.

Figure 4: Schematic of an Arbitrary Sintering Cycle

3.2.2.1 Delubing stage

The delubing stage is a consequence of the lubricant or binder required during conformation of the powder metallurgy part, but neither the lubricant or binder are desired as part of the piece’s composition during sintering due to negative consequences of the final properties. Therefore, if hydrocarbons are used, the function of the delubing stage is to burn and remove hydrocarbons4.

The organic material surrounds the metal powders in a homogeneous distribution within the piece. Organic material will be removed via the paths that develop from the outside inwards because the organic material on the outside will be the first to react and evaporate. The canals left serve as

4 Delubing also assists in clearing undesired surface contaminants.

11 evacuation routes for the lubrication contained deeper in the piece. Since the organic material exits as gas, care must be taken to avoid entrapping an excessive amount due to insufficient evacuation rates because an excessive amount of gas will cause an internal explosion rupturing the piece. Finally, if all goes well, the evacuated organic material leaves behind a network of pores where organic material once assisted with the green strength of the compact. The previous occurs only when the proper control of holding times, temperatures, and flow rates are achieved. At this moment, between the delubing stage and the preheating stage, the extraction of surface contaminants and processing organics can be verified, tentatively, via thermogravimetric data on weight loss.

Delubing temperature is governed by the type of lubricant used. Four commonly employed compacting lubricants are Acrawax, lithium stearate

(LiC18H35O2), paraffin (C22H46 to C27H56), and zinc stearate (Zn(C18H35O2)2). Their melting points respectively are: 140-143°C, 221°C, 40-60°C, and 130°C. So an acceptable range for common lubricants can be considered between 400°C to 650°C since a gaseous phase is desired and not a liquid phase5. A gaseous phase facilitates extraction of the organic material in a scenario of increasing difficulty of removal when green compact density increases.

Make note, explicitly, that the acceptable range of temperatures imply that the transition from melting ranges to temperatures in evaporation ranges cause part of the organic material to be driven out in a liquid phase. The extraction of a liquid lubrication continues until reaching the selected delubing temperature where the rest and most of the material leaves in a gaseous phase.

5 T his might pose a problem since at this temperature the stainless steel recipient is susceptible to being sensitized and thus leading towards stress corrosion cracking.

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It is desired to be at the verge of the upper limit of the permissible temperature range due to reduced time requirements. But the upper limit temperature for any particular organic material is a compromise of the maximum temperature delubing can be performed at without the thermal degradation of the lubricant or binder. If the chemical degradation occurs, the organic material can react undesirably with the piece. Another possible danger of operating at the upper temperature limit is the rupture of the piece induced by stresses involved in accelerated volumetric changes (thermal stress rupturing) when the hydrocarbon leaves too promptly.

Delubing also includes the removal of the burnt material. The removal of the material may require assistance from the atmosphere at this stage. Small amounts of oxidants (e.g.: water, carbon dioxide, or an atmosphere with high dew points) are used. Therefore, a reaction of the following type (equation [1]) is expected with oxidants in the atmosphere:

C x H y + H 2O → CO +CO2 + H 2 [1]

Atmospheres leading to the previous reaction permit the hydrogen and carbon to leave the piece as gases. When oxidants are denied, for example, carbon would possibly remain as a sooty product. Once the transition to a gaseous product is achieved, the residue must be channeled out of the tubular recipient before proceeding to the next stages of the sintering process. If it is concluded that delubing is successfully performed, the now even more fragile, porous green piece is ready to proceed further in the sintering process.

13 3.2.2.2 Preheating stage / Presintering

The preheating stage, also referred to as the presintering stage, prepares the piece for the diffusion activity that will occur during the hot stage. Diffusion requires atomic migration. So, for example, oxides in steels can impede drastically diffusion activity. Therefore, the preheating stage may be used for removal of surface oxides on the powder.

Another consideration pertaining to the preheat stage is the rate of temperature change of the piece. The piece must reach the final hot stage temperature starting from the lower delubing temperature. It is obvious that the piece will expand during the transition. If the expansion generates tension stresses higher than the combination of the low resistance strength offered by the weak bonds / links left by the capillary channels and the new, scarce bonds forming during diffusion at this stage, the piece will rupture. Thus, it is key that the preheat stage consider the rate of the incremental temperature transition.

3.2.2.3 Hot Stage

Temperatures may be around 60% of the melting point of the metal. At this stage, the diffusion activity is at its highest rate. Thus, it is at this stage that most of the densification occurs. And since the temperature is at the highest of all of the sintering cycle, there will be more available energy for reactions to proceed. The high energy available may lead to undesired reactions of oxidation or reduction of the piece and the equipment. Therefore, the practitioner must apply a controlled atmosphere as needed.

14 3.2.2.4 Cooling Stage

The main problem associated with the cooling stage is time consumption. Tim e cons umption, relatively s peaking, is put into pers pective when viewed from the length of the cooling zone compared to the sintering zone in a continuous furnace. The proportion may be of up to two and a half times. Reduction of these extended times are possible by managing cooling rates.

Cooling rates are manageable when assisted by forced methods. Methods such as heat transfer by convection with circulating gases from the atmosphere around the piece easily reduce lengths in the cooling zone by 50%. And if these cooling rates are high enough, a sinter hardening process may even become available if required.

Additional considerations of the cooling stage is the cooling rate uniformity of the load and the safe discharge of the load into standard atmospheric conditions. Cooling rate uniformity throughout the load depends on the size, shape, quantity, and distribution of what is being processed. Therefore, the placement of parts for cooling inside the retort, determine how well the part cools or heats. And secondly, before the piece can be discharged into an atmosphere with air, the temperature needs to be low enough to prevent reaction with the air.

3.2.2.5 Slow cooling stage

At the end of the sintering cycle is the slow cooling stage. Basically, it consists of letting the pieces to cool without assistance (just normal atmospheric conditions) until reaching manipulation temperatures. It is important to open the recipient commencing the slow cooling stage ONLY if

15 the atmosphere inside is safe to mingle with the exterior conditions and the properties of the pieces are uncompromised.

3.2.2.6 Purging, a used as needed stage

Purging is an intermediate stage. It is sometimes required before beginning the sintering process or between any of the stages when there is a change of atmosphere. For example, purging of air from the retort before the furnace is heated to temperature above 150°C may help prevent oxidizing the interior of the retort or the pieces. Yet, the possibility of an intentionally oxidizing atmosphere does exist. In other words, purging is not a rigorous stage that is always performed.

Purging can be performed simultaneously with a temperature change and it can be done as quickly as a practical flow rate allows. Mainly, a “safe” atmosphere is sought with purging before reaching temperatures that permit proceeding of undesired thermo-chemical reactions. The atmosphere is “safe”, as a general rule of thumb, by replacing the contents of the control volume by five times.

Purging, particularly concerning this project, is used to eliminate the presence of air, oxygen, or other oxidizers prior to admitting hydrogen into the systems; and, inversely, the system is purged of hydrogen before opening the system to the atmosphere. Purging should be done to prevent the formation of flammable mixtures and can be accomplished in several ways. The system should be “inerted” by suitable purging, for example, with nitrogen.

Exactly how purging is achieved is left to the designer of the procedure for sintering based upon requirements. Roughly, the designer must pick set-

16 points of time and temperature to initiate and terminate purging. The designer must additionally consider safety hazards, purging techniques, and know consequences of the selections.

3.2.3 Parameters

Certain variables need to be set according to the sintering process needed by the green parts. Adverse effects are the outcome of adjusting temperature profiles and adjusting chemical composition of the processing gas. Therefore, some knowledge must be known about these parameters.

3.2.3.1 Temperature – Heat

The interaction of the retort with the external heating system defines not only temperature gradients within but also the type of forced and particular response to external inputs. One of the mayor goals of the characterization is obtaining the transitional and stable response in time of the retort system. Basically, the system’s homogeneous response to the inputs of heat at the external surface of the retort is likely a first order type. Yet, this prediction may be wrong.

Heat flow is one of the variables that must be comprehended. There are three places where heat transfer are a concern. The first is the heat transfer from the furnace to the retort; the second is the heat losses which increase power consumption of the furnace; the third is the heat transfer to the green pieces. But even though the need of understanding heat has been identified as important, a realistic approach must be taken because theoretical models can be very complicated, as the following example will show. After the explanation is given, the alternative empirical method will be developed.

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Take for instance the heat transfer to the green pieces. How does it get there? The green piece is placed on top of a ceramic support to keep it from welding to any structure. So the support is nonconductive; therefore the piece bottom will heat mostly by conduction from exposed sides of the piece to the bottom. A little will be accomplished by the gas interaction which effuses through the pores of the support.

By taking a step back and going to the more important heat transfer to the walls of the piece, an additional complication is identified. At first most heat is transferred via convection. Convection depends on numerous fluid properties such as density, viscosity, thermal conductivity, specific heat, surface geometry, and the flow conditions. This multiplicity of independent variables result from the fact that convection transfer is determined by two boundary layers that develop on the surface. The first being the velocity boundary layer and the second is the thermal boundary layer. Both boundaries consider the average properties. For example, the velocity of travel of the fluid is independent whether the motion is turbulent or laminar. Convection heat transfer, consequent of a thermal boundary layer, only occurs if there is a difference between a surface and free stream temperature. And this could go on and on. Matter of fact, I recommend the text book “Introduction to Heat Transfer” because convection is only relevant at lower temperatures. At higher temperatures radiation contributes most of the heat transfer. As a consequence of the complicated extent of the theoretical approach to evaluate heat, deductions will be based on empirical values.

Thermoelectric means (i.e. thermocouples) based entirely on empirical calibrations is suitable for temperature measurement. The empirical calibrations are accompanied by the application of so-called thermoelectric

18 “laws” which experience has shown to hold. Please see the annexed section that includes the Table: Summary of Thermocouples .

With the use of properly selected thermocouples, in this case type K, temperatures will be recorded with a SIX CHANNEL DATA ADQUISITION CIRCUIT. A direct, brief description is given of the how the data acquisition works in the section: Six Channel Data Acquisition.

3.2.3.2 Gases

Gases present during the sintering cycle are an essential part of the atmosphere because the selection of the gases for use inside the retort is directly responsible for the proper and safe operation of the equipment and the quality of the powder metallurgy products. This means that a prior understanding of gases is necessary to continue the development of this project.

3.2.3.2.1 Classification of Gases

Gases are classified as oxidizers, inert, or flammable. The manner in which they contribute in combustion defines which category they belong to. Oxidizers, in effect, will contribute to combustion as an oxidant, but they are not flammable by themselves. Inert gases do not participate in combustion processes since they do not react with other materials. Instead of contributing in combustion, inert gas can limit a combustion process in a limited space by reducing the amount of – for say – oxygen. Flammable gases, together with air or oxygen in the right concentration, if ignited will burn or even explode. The following table classifies some common gases under their respective types:

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Table 2: Classification of Some Common Gases

Oxidizers Inert Gases Flammable Air Argon Acety lene Chlorine Carbon Dioxide Ammonia Fluorine Helium Arsine Nitric Oxide Neon Butane Nitrogen Dioxide Nitrogen Carbon Monoxide Oxy gen Xenon Cyclopropane Ethane Ethylene Ethyl Chloride Hy drogen Isobutane Methane Methyl Chloride Propane Propy lene

3.2.3.2.2 Combustion (Explosion) of gases

Combustion or explosion only occurs if the mixture is not too lean or too rich. The limits of a lean mixture and a rich mixture are the limiting concentrations commonly called the “Lower Explosive Limit” (LEL) and the “Upper Explosive Limit” (UEL)6, respectively. Within this range, a gas or vapor concentration will burn or explode if an ignition source is introduced. Below the explosive or flammable limit the mixture is too poor to burn and above the upper explosive or flammable limit it is too rich to burn.

Therefore, even if a gas is considered flammable, mixtures with oxidants will only burn if the fuel concentration lies within sharply defined LEL and UEL. Outside of these limits ignition and flame propagation can not be initiated by the application of an external stimulus. Even should a reaction mixture lie within its flammability limits ignition requires the input of sufficient energy in a suitable form.

6 Alternative names for the “Lower Explosive Limit” and the “Upper Explosive Limit” are “Lower Flammable Limit” and the “Upper Flammable Limit” respectively.

20 It should be noted that the phenomenon of flammability is distinct from, although related to, auto-ignition. The auto-ignition temperature is the lowest temperature at which the spontaneous ignition will occur of a mixture between a flammable with an oxidant without an ignition source.

3.2.3.2.3 Ignition (Auto-ignition) of gases

A process where a mixture does not ignite by itself but by a local ignition source is called induced ignition. In induced ignition, energy is deposited locally leading to a temperature rise in a small volume of the mixture. Then, from then on, auto ignition may take place generating more radicals. Flame propagation continues setting the remaining mixture on fire.

For safety reasons, the ranges of temperature, pressure and composition a mixture can auto-ignite should be known. Two possibilities exist within the ranges. The first, a mixture will ignite spontaneously. The second possibility, only a slow reaction is observed.

When a flammable-oxidant mixture is supplied with sufficient energy, it still will not ignite until an induction time (ignition delay time) has passed. This ignition delay time can be as long as several hours or as short as microseconds and is characteristic for radical-chain explosions. During this time span, the radical population increases exponentially. These chemical reactions, radical formations, do consume fuel but the temperature remains nearly constant. As soon as the radical pool has grown enough to consume a significant fraction of the fuel, ignition occurs and the temperature starts to rise. In contrast, in a purely thermal ignition process there is no induction time, and the temperature increases immediately.

21 Combustion processes involve radical chain reactions. Chain initiation steps start the reaction. In chain propagation reactions, the number of radicals does not change. It is the chain branching reactions that lead to an exponential increase in the radical pool. Chain termination can occur in a homogeneous or inhomogeneous manner.

Examples of these types of chain reactions are:

- chain initiation: H2 + O2 = 2OH

- chain propagation: OH + H2 = H2O + H

- chain branching: H + O2 = OH + O

- chain branching: O + H2 = OH + H

- chain termination (heterogeneous): ½ (H+ H) = ½ H2

- chain termination (homogeneous): H + O2 + M = HO 2 + M

3.2.3.2.4 Gases for the atmosphere

The gases chosen for the different stages of sintering are according to the assisting functions of the interaction amongst the atmosphere with the piece. In each stage the needs differ, thus the vapor phase surrounding the piece as the sintering process progressed must be modified.

Examples of some common roles of the atmosphere of particular interest for this recipient include:

• Accelerate removal of the lubricants or organic bonders at the low temperature stage of delubing.

22 • Guarantee the removal of organic products from the recipient in a gaseous phase; extraction of organic products in a liquid or solid phase are considered a nuisance and unachievable. • Reduce unnecessary oxides of the compact which impede proper sinter bonding in the preheat stage. • Provide inert and neutral atmospheres; uncontrolled amounts of oxygen and air will produce undesired reactions during the hot stage. • Allow the necessary partial pressures for the selected combination of atmospheres throughout any sintering stage. • Hydrogen and nitrogen are the two particular gases initially in mind for the process. The properties and any pertinent information will be described here after. And keep in mind throughout the development of this project how the design is based on implications of the gases used.

3.2.3.2.4.1 Hydrogen

Hydrogen is commonly described as:

“Hy drogen (H2) is a colorless, odorless, flammable gas which may ignite spontaneously and burn with a colorless flame. Hydrogen is the lightest gas known and it is normally compressed and shipped at high pressure.” [Ref. 23]

Operating precautions are necessary because hydrogen is flammable according to the classification of gases. Therefore, operating precautions with hydrogen might address combustion hazards, pressure hazards, low temperature hazards, hydrogen embrittlement hazards, purging capability of the equipment, and health hazards. Pressure and low temperature hazards are obviated since the conditions of this project never place hydrogen in a cryogenic/liquid state. In effect, the lowest expected temperature is approximately 5 °C when hydrogen is stored as supplied in the high pressure

23 cylinder. Consequently, only a brief consideration of combustion hazards and hydrogen embrittlement hazards are presented. Safety hazards can be reviewed in the hydrogen MSDS.

If the reader desires to access basic but sufficient information for handling hydrogen he or she should refer to the MSDS (Material Safety Data Sheet) included as the file “Hydrogen MSDS” under the references folder of the annexed compact disc. Furthermore, reading of the presentation of “HYDROGEN HANDLING SHORT COURSE”, prepared by Stephen S. Woods and found in the file HSCWoodsGeneral, is highly suggested.

But in general, due to hydrogen, the following safety precautions have been applied: • The retort should never be opened with the presence of hydrogen above 150°C. • Purge prior, as required, if the necessity of opening the retort arises. • Vents should be located to prevent hydrogen from impinging on ventilation ducts or other equipment.

3.2.3.2.4.1.1 Combustion Hazards

Hydrogen is only flammable and even explosive with the presence of an oxidant. Hydrogen is dangerous when certain compositions by volume percentage with an oxidant are met. The limits are summarized in the table that follows.

Table 3: Limits of Hydrogen by Volume Percentage

Flammable Limits @ 1 [atm] Detonable Limits @ 1 [atm] In Air In oxygen In Air In oxygen Lower % Upper % Lower % Upper % Lower % Upper % Lower % Upper % 4.00 74.2 4.65 93.9 18.2 58.9 15 90

24 Even if the atmosphere composition contains other constituents besides hydrogen and the oxidant, it is only necessary to compare the volumes of the hydrogen and the oxidant. But even when hydrogen content does fall within the explosive range, the possibility of explosion is present only if an energy source is present. An energy source can be a spark or heat. Therefore, all equipment when operating in presence of hydrogen should be grounded.

When the energy source is of the type of a spark, an extremely small amount of power is needed to start a reaction (i.e. the minimum amount is 0.017 [mJ]7). The amount of power needed is comparable to the power available from static electricity. And without a spark, auto-ignition of hydrogen within an oxidant can also occur. For example, auto-ignition in air is commonly taken as 500 °C.

3.2.3.2.4.1.2 Hydrogen Damage

This section could be placed with or after the Retort Construction section since hydrogen damage should be studied according to the retort material. But at this point, it must be admitted that this section was modified and created taking advantage of the feedback loops described in the flowchart of the Tubular Recipient Development Model figure. The material of the retort has already been selected as a stainless steel 304.

Particular hydrogen effects on this stainless steel had to be considered. Since hydrogen is the smallest atom and at high temperatures is in its monatomic structure, the possibility of hydrogen attack and hydrogen embrittlement are considered. Fortunately, those two particular forms of damage caused by hydrogen are mostly associated with low-alloy or carbon steels. Yet a description is included for general knowledge.

7 Higher power demands for ignition are required at different percentage volume compositions and conditions, but for safety’s sake, the minimum is considered.

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Hydrogen attack refers to the mechanism of damage of hydrogen when at temperatures above 220 °C penetrates the structure. The hydrogen then reacts with reducible species such as an iron carbide phase. The product of the reaction is methane which is then trapped inside the structure because the size of the molecule is too large to diffuse back out. Press ure builds as a consequence of the methane gas trapped inside the structure. The pressure builds until fissuring the steel. Once again, hydrogen attack does not occur in austenitic stainless steels. [Ref. 9]

Hydrogen embrittlement, on the contrary of hydrogen attack, does affect stainless steel 304. But due to the nature of the function of the retort being static with low stresses, embrittlement is not a problem. Furthermore, depending upon hydrogen exposure of the stainless steel 304, tensile strength decreases and the yield strength increases (as seen in the immediate figure). If it does become a concern, a bake-out cycle with temperatures between 175 °C and 205 °C allows the hydrogen to escape the metal. [Ref. 9]

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Figure 5: Stress-strain curves for 304 stainless steel at vary in hydrogen concentration (given in wt%) [Ref. 8] What is of great importance to austenitic stainless steels is hydrogen stress cracking. The explanation of this mechanism is left for the section Retort Construction – Shell.

3.2.3.2.4.1.3 Commercial availability of hydrogen

Hydrogen is provided commercially in Bogotá with purity levels over 99.9%. The hydrogen used for this project is supplied by OXIGENOS DE COLOMBIA PRAXAIR INC. with a minimum purity of 99.99% and with a price of 35,000 [COP/mt3] std. for students. For non students the price is 42,000 COP if the right person is contacted.

Thus, the hydrogen of interest is the following:

Grade: 4 Minimum purity: 99.99%

Levels of impurities: H2O < 10 ppm; O2 < 5 ppm Total Hydrocarbons < 0.5 ppm

27 OXIGENOS DE COLOMBIA PRAXAIR INC. facilitates rental services to their clients. They offer the following high pressure gas cylinder suitable for containing hydrogen with the following specifications:

Table 4: Gas Cylinder Specifications

Operation Hy draulic External Total Length Weight Model Pressure Capacity Diameter bar kpsi liters cu in mm in mm in Kg lbs 40.219.150 150 2.176 40.00 2,440.80 219.00 8.62 1,320.00 51.96 51.00 112.46 MeMn

The line out port for the particular hydrogen cylinder has a CGA 350 connection. Additionally, the exterior of the cylinder is painted for identification with bright red. Some pertinent information about safety for clients using high pressure cylinders is available under the Gases folder included in the reference folder of the compact disc with the file name “BOLETIN DE SEGURIDAD PARA CLIENTES”.

Rental fee for the cylinder is 400 pesos daily. A transportation fee of 4000 pesos must also be paid.

3.2.3.2.4.2 Nitrogen

Nitrogen is used for purging and as a “filler gas” due to its inertness. A “filler gas” is simply present during operation as part of the process mixing gas to eliminate the potential hazards of hydrogen. Nitrogen should not affect negatively the equipment. If it does diffuse into the structure, it might even help impede sensitization of the stainless steel.

28 The nitrogen of interest is the following: Grade: 4.6 Minimum purity: 99.996%

Levels of impurities: H2O < 5 ppm; O2 < 5 ppm

3.2.4 Sintering Equipment

Selection of the appropriate furnace for the needs of the powder metallurgist can be based on several types of criteria. Possible criteria are: initial cost of equipment, production cost per weight – quantity, physical characteristics of the green pieces (e.g. weight), maximum operating temperature, and capability of filling and containing controlled volumes with the necessary atmosphere. But in general, the broadest two categories furnaces can be divided into are continuous and batch.

The continuous furnaces are for larger productions and hence the pieces are conveyed through the furnace at constant rates. The batch furnaces are for smaller productions and are usually placed manually or in a discrete manner. Both of the type of furnaces are subdivided into subcategories and they all posses relative advantages and relative disadvantages. But even if there are subdivisions, one characteristic holds for all subdivisions of the two categories.

All continuous furnaces have separate chambers for each stage of the sintering process. The loads are convoyed through each chamber to complete the process. For the batch furnace category, all of the process is performed in a single multipurpose chamber. And of course, there are many combinations of sub-systems in all sintering equipment.

29 Table 5: Types of Batch Furnaces Type Description Bell “The base is permanently installed on the floor. Work to be processed is loaded onto the base and then covered with a heat- resistant alloy retort to contain the protective atmosphere. The furnace bell then is lifted and placed over the retort and base.” [Ref. 5] Elevator “It has the furnace bell above the mill floor on a fixed structure. The base and work load are covered with a retort and rolled on tracks under the furnace. An elevator system then raises the base, work, and retort into the furnace.” [Ref. 5] Vacuum “A vacuum furnace operates in the absence of an internal atmosphere.” “Generally, a batch-type vacuum furnace consists of an outer vacuum-tight cylindrical casing that contains a furnace with radiation shields or other types of insulation, work support, and heating elements. This casing is fitted with roughing and diffusion pumps to achieve the desired vacuum levels.”[Ref. 5]

3.2.4.1 Batch Furnaces

This type of furnace may be used when low quantities are produced and when the sintering process requires various heat treating conditions. It basically consists of a controlled volume (also called a retort), a heating unit, a base, a work support, a fluid flow unit, and a bell-shaped or cylindrical furnace.

The three common and standard furnaces are the bell, elevator, and batch- vacuum furnace. The descriptions are as given in the table: Types of Batch Furnaces.

3.2.4.2 Continuous Furnaces

Continuous furnaces are usually used in productions of large quantities. They tend to be more automated than the batch furnaces. Also, they tend to

30 be more expensive than the batch furnaces. Following is a brief description of the main types of continuous furnaces.

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Table 6: Types of Continuous Furnaces Type Description Mesh-belt Conveyer “Mesh-belt conveyor furnaces consist of a charge and belt-driven table, a slow cooling zone, a final cooling zone, and a discharge table.” “These are the most commonly used equipment for the sintering of P/M compacts. They provide a continuous, reproducible time-temperature-atmosphere (thermal) profile.” [Ref. 5] Humpback “This is adaptation of the mesh-belt conveyor are used when high atmosphere purity and low atmosphere consumption are desired…” “A long, inclined entry section, which is gas tight, carries the belt and work from the charge table up to the sintering zone.” [Ref. 5] Roller-Hearth “In a roller-hearth furnace, parts are carried in trays through the furnace on driven rolls. Generally, these furnaces are capable of heavier loading on the hearth than mesh-belt furnaces.” [Ref. 5] Walking-beam “These are particularly well suited for applications in which sintering temperatures are above the limitations of the mesh-belt conveyor and roller-hearth furnaces.” [Ref. 5] For movement, a beam pivoted as a four bar mechanism creates rectilinear motion and displaces the pieces forward discretely. Pusher Furnaces “In a pusher furnace, parts to be processed are loaded on trays or ceramic plates that are pushed through the stationary hearth furnace.” “The pusher mechanism may operate intermittently or continuously.” [Ref. 5] Vacuum “The furnace is comprised of an external loading table, followed by an atmosphere delubrication chamber, a transfer station, and an “atmosphere to vacuum” vestibule section, followed by heating chamber, vacuum cooling chamber, a combination fan cooling and “vacuum to atmosphere” vestibule, and an unloading table. The operation of the furnace is completely automatic, under the supervision of an operator.” [Ref. 5]

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3.2.4.3 Heating Units

For both continuous and batch furnaces, the heating units are the traditional heating units. They are the direct-fired gas systems, radiant tubes for indirect-fired gas systems, and the electrically heated with resistance elements.

The direct-fired systems are usually the cheapest system to fuel. The fuel of the equipment can be natural gas, straight propane, a propane air mix, or any grade of oil that can be atomized. Since the products are exposed directly to the products of combustion they are referred to as flue products. These flue products are often not in their final stage. For example, if oxides are created superficially, it might not be a problem because the part might proceed into an additional stage of final sizing. At the final sizing, the scales could be removed.

The disadvantage possessed by the direct-fired systems are the need of a pressure-control system due to the flue created from combustion. The flue produced is difficult to control and has adverse consequences on the products. Therefore, only certain materials or types of products can be sintered in direct-fired furnaces.

The indirect-fired gas systems heat the pieces mostly by radiation. The tubes are heated from the inside either by gas combustion, oil combustion, or electrically heat produced from resistances. Then the heat travels by conduction through the tube and is radiated outwards. If the tubes use gas or oil, the tubes protect the pieces by containing combustion within protective tubes. Thus, the products of combustion never interact with the pieces and there can be controlled atmospheres around the work as dictated by the sintering process. If the tubes are heated electrically, more then likely the

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tubes are in place to protect the resistance elements from the furnace atmospheres.

The electrically heated furnace equipment are common in all temperature ranges. Like it was stated previously, they can be placed within tubes and be considered indirect or they can be exposed and be considered direct. Factors that influence the decision of direct or indirect depend a whole lot on the interaction between the elements and the furnace atmosphere. For example, the furnace atmosphere can cause mechanical damage to the elements. In any case, part of selecting an electrically heated furnace is choosing the element’s material (metallic or non-metallic), the configuration of the resistance (e.g. wire diameter or pitch between coils), and an electronic controlling device. Clearly the advantages of an electrically heated furnace over direct-fired or indirect-fired equipment are the system’s cleanness, ease of controlling temperature cycles automatically and consistently, and uniform heat distribution.

3.2.4.4 Gas Pressure and Flow Equipment

Pressure is the first variable to set in the fluid flow system’s lines of the processing gas that enters the retorts. The second variable to set is flow rate. And for a flow to exist through a flow metering device, there must be a pressure difference either upstream the metering device or at the metering device itself. But if the pressure at the entrance of the metering device drops continuously, a constant, downstream flow cannot be kept unless the flow metering device is choked. Thus, a pressure difference is created upstream with pressure regulators and then the flow is regulated.

A pressure regulator’s primary function consists in reducing high-pressure gas in a cylinder or process line to a lower, usable level as it passes to other

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IM - 2005 - I - 39 equipment. They also maintain pressure within a system. However, a regulator is not a flow control device. Its only function is controlling delivery pressure.

There are four main types of gas pressure regulators determined by their specific application function. These are line regulators, general-purpose regulators, high purity regulators, and special service regulators. A combination of a line regulator in series with a high pressure, general purpose regulator is of particular interest. Typically, a line regulator is the point-of-use regulator serving in low pressure lines after the pressure is reduced by a general purpose regulator to an inlet pressure adequate for the line regulator.

The scenario for this project is gas that exits from the cylinder source initially at approximately 2000 p.s.i.. As the source is depleted, there will be a continuous pressure drop until the pressure inside the cylinder is equal to the downstream pressure. And the flow of the gas will cease when the pressure equilibrium is reached. This means that a constant pressure difference must exist between two particular points.

Say point A is located at the exit valve of the cylinder and B is just before the entrance of the fluid metering flow device. After point A, the general purpose regulator reduces most of the pressure to an intermediate point between point A and B. And then, the gas flows into the line regulator. The pressure drop across the line regulator is approximately a tenth of the pressure drop across the general purpose regulator. The reduced pressure drop difference across the line regulator lets it make fine adjustments when pressure varies downstream. Therefore, the line regulator tries to correct its outlet pressure when fluctuations downstream occur. This might prove of extreme usefulness as the gas inside the recipient might experience an increase in pressure due to the temperature rise.

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There must be only a slight pressure difference between the inlet line of the retort and atmospheric pressure. This pressure must be set such that the metering flow device is calibrated against it. And the amount of flow is determined by situations, for example, such as those needed to evacuate water vapor formed during the reduction reaction between the hydrogen and oxides (or air) of the sintering cycle.

There should always be evidence that in fact there is a flow. If the flow ceases, the equipment could be damaged by oxidation; or the worst case scenario, if the flow of nitrogen ceases and the hydrogen flow does not, a combustion hazard would exist. So a variable area flowmeter is installed after the metering flow device. The accuracy standards are given in the table below.

Table 7: Accuracy of Variable Area Flowmeters

Accuracy class 0.4 1.0 1.6 Total error % Measured Full-scale Measured Full-scale Measured Full-scale 100 0.400 0.400 1.000 1.000 1.600 1.600 90 0.411 0.370 1.028 0.925 1.644 1.480 80 0.425 0.340 1.063 0.850 1.700 1.360

70 0.443 0.310 1.107 0.775 1.771 1.240 60 0.467 0.280 1.167 0.700 1.867 1.120 50 0.500 0.250 1.250 0.625 2.000 1.000 40 0.550 0.220 1.375 0.550 2.200 0.880 Flow rate %

30 0.633 0.190 1.583 0.475 2.533 0.760 20 0.800 0.160 2.000 0.400 3.200 0.640 10 1.300 0.130 3.250 0.325 5.200 0.520

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Accuracy class 2.5 4. 0 6.0 Total error % Measured Full-scale Measured Full-scale Measured Full-scale 100 2.500 2.500 4.000 4.000 6.000 6.000 90 2.569 2.313 4.111 3.700 6.167 5.550 80 2.656 2.125 4.250 3.400 6.375 5.100

70 2.768 1.938 4.429 3.100 6.643 4.650 60 2.917 1.750 4.667 2.800 7.000 4.200 50 3.125 1.563 5.000 2.500 7.500 3.750

Flow rate % 40 3.438 1.375 5.500 2.200 8.250 3.300

30 3.958 1.188 6.333 1.900 9.500 2.850 20 5.000 1.000 8.000 1.600 12.000 2.400 10 8.125 0.813 13.000 1.300 19.500 1.950

3.2.4.5 Additional Comparisons

As stated initially, there are several ways on the selection of furnaces. Here are some direct comparisons.

Table 8: Operating Characteristics and Capital Cost of Sintering Furnaces [Ref. 17] Type Maximum Capital Pounds Parts Operating Cost Produced Temperature °F $1,000’s Per Hour* Mesh Belt 2100 $275 450 Ceramic Belt 2400 $350 250 Roller Hearth, 36” 2200 ** 2000 Pusher >2400 $375 250 Walking Beam >2800 $750 500 Continuous Vacuum >2400 $850 350 * Production rate depends on the ability of parts to be packed closely enough to yield this rate. ** Capital cost is about 50% more than

3.2.4.6 Thermocouples

Temperature can be measured via a diverse array of sensors. All of them infer temperature by sensing some change in a physical characteristic. Six common types are: thermocouples, resistive temperature devices (RTDs and thermistors), infrared

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IM - 2005 - I - 39 radiators, bimetallic devices, liquid expansion devices, and change-of-state devices. For this project, thermocouples will be used.

Any two dissimilar metals joined together is a thermocouple, but there are seven standard thermocouples. Of the seven, a type K is selected. A type K thermocouple is suitable for sintering because it functions within a temperature from –200 °C up to 1250 °C.

It consists of two wires joined together at a junction. One wire is an alloy of nickel- chromium and the other wire joined is an alloy of nickel-aluminum. The given limits of error of standard type K thermocouples are 2.2 °C or 0.75% above 0 °C and 2.2 °C or 2.0% below 0 °C. Furthermore, this thermocouple cannot be exposed to a reducing atmosphere. Revised thermocouple reference tables are found in the Omega website. The tables contain the values of thermoelectric voltage in millivolts corresponding to temperatures.

ALL INFORMATION PERTANENT TO TEMPERATURE MEASUREMENTE HAS BEEN OBTAINED FROM [Ref. 22].

4 Methodology

After reading this chapter, an orderly development of this project should be sensed. So as the reader advances, he or she should understand how each part of this document contributes and fits within the global contents for accomplishing the objective.

A tubular recipient, which is the shape given to the retort, is an equipment within which green samples created with powders are placed so they can be sintered. The energy for sintering comes from the furnace where the

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IM - 2005 - I - 39 recipient is placed. Thus a furnace is needed for this project. It has opted to only design the recipient and not the furnace. So this project utilizes what is already available at the University of Los Andes. This implies that the retort functions within the furnace described below.

Thermolyne Type 6000 Furnace Volume 864 in3 Interior Depth 10.0 in Interior Height 6.8 in Interior Width 12.8 in Power 3905 watts

Figure 6: Furnace Equipment [Ref. 19]

This furnace is used as a batch furnace. Though another project could actually design a recipient with movement into and out of the furnace making it a continuous furnace system. Additional to the physical properties given, the furnace has been adapted with a Eurotherm Controller/Programmer Type 818 and the lid has been replaced with a permanent, thermally isolated lid with a opening large enough for the retort.

The recipient designed is sealed, has a fluid flow system, and a measurement system for temperature. By referencing to the figure: Virtual Model, it is seen how the sintering system is composed of a 3 inch nominal diameter, schedule 40 tube which slides into the furnace.

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Figure 7: Virtual Model

One end of the recipient is sealed permanently and the other end uses a blind flange as a lid (not shown). The open end is used for loading and unloading the green powder metallurgy parts that are placed on ceramic supports (shown as a yellow cylinder). The ceramic supports slide on the gray tray. The two purple tubes are the lines in and out of the processing gas mixture. Once the pieces are loaded, the lid not shown is fitted flush against the flange and held in place with a sanitary clamp (none of which is shown). Also, the lid has fitted the measurement thermocouples within sheaths. The whole apparatus slides into the furnace for the sintering cycle. [Ref. 12], [Ref. 14], [Ref. 5], [Ref. 11]

To reach the preliminary model, a knowledge of requirements encompassed the starting point. The knowledge included a general understanding of powder metallurgy. Thus, the sections Overview of Powder Metallurgy, Overview of Sintering, and Schematic of Creating a Sintering Cycle are included as answers to requirement that must be met by the retort. From those sections it is concluded that if the development of the recipient permits control and monitoring of time, temperature, and atmospheres, which are the

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parameters of the sintering process, a high quality powder metallurgy product is resultant and the project is a success. [Ref. 12], [Ref. 14], [Ref. 15], [Ref. 11]

With the requirements defined, three distinct sub-systems of the retort system (consider the retort equipment as a system) are identified: the physical system, fluid flow system, and thermal system. Each of these are separately developed. In all three systems, key components and variables are discretely considered for future, individual design. This is the motive behind the sections of Gases and Diffusion.

The section of Details of Creating a Controlled Atmosphere then explicitly unites the solutions given to the three systems. At this point a design for a measurement system is required for evaluation of the solutions given. The measurement system is a combination of mechanical and electronic data acquisition. And this would conclude the first part of the objective as the preliminary design stage. Logically, the construction stage proceeds, and with all of the previous completed, design and construction, tests and interpretation and characterization of the results follow.

It is important to note that all times there is feedback, but the causes of the feedback are not included. Instead, the results are given.

Finally, the development of the project is given below in figure 3 as a flow diagram for clarification.

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Fi gure 8: Tubul ar Recipient Devel opment Model

5 Results and Analysis

Design of the sintering equipment became possible with all of the background information. The results of the different areas of the equipment are given in this chapter. Afterwards, a brief analysis is based on the results of the characterization experiments.

5.1 Details of Creating the Retort

The concept of a controlled atmosphere consists of being able to measure and control the parameters within the retort. Temperature is one of the parameters. Quantity and composition of the processing gas are other parameters. These three parameters must be measurable at all times. And the retort system must also allow adjustment of the parameters.

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The figures displayed under Figure: Prototype are pictures of the equipment.

Figure 9: Prototype

5.1.1 Six Channel Data Acquisition

A Six Channel Data Acquisition electronic equipment was designed for particular use of collecting data for the retort. Basically, it is designed to collect information from transducers which can be for pressure, flow, or temperature. Initially, the data collection of this project is limited to the voltage signals given by the thermocouples and converting to degrees Celsius via computer software.

The figure Data Acquisition Card shows a picture of the result.

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Fi gure 10: Data Acqui si ti on Card

5.1.1.1 General Architecture of the System

Input: -Temperature of environment -Mode of operation selected by the user: Start Data Acquisition, End Data Acquisition, Display report with Excel, Sampling Time Output: -Led indicator when the state is ON -Display of temperature of each thermocouple -Report obtained data on Excel

Figure 11: General Architecture of System

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5.1.1.2 Block Architecture of Temperature Sensors

Inputs : -Signal from six type K thermocouples tipo K -Command of Start Data Acquisition and number of thermocouple of the desired thermocouple. Outputs : -Digitalized value of selected channel

Figure 12: Block Architecture of a Temperature Sensor

5.1.1.3 Implementation

General Control Block, Control of Temperature Sensors, and Multiplexing: After revising data acquisition possibilities, it was concluded that the opt technology for implementation is through microcontrollers. Then a choice amongst the microcontrollers available was based on the following selection criteria:

• A minimum of at least six channels • At least a 10 bit converter • An acceptable conversion time below half a second

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• Simplified programming • Commercial availability in Bogotá • Of low cost.

The criteria lead to the selection of a PIC18F452 microcontroller fabricated by Microchip. The datasheet is included in the Reference folder under the Data Acquisition Folder.

Block of Amplification Signal: This block is implemented with an op-amp in a non-inverting configuration. The amplification is approximately of 100x. The component used is a LF353 with the necessary amount of precision in amplification. Resistors of 100 [Kohms] and 1 [Kohms] are implemented.

Block of Analog to Digital Conversion: The ADC of the PIC18F452 is used.

Communication Interface with the PC: A DLP-USB245M fabricated by FTDI chip permits Parallel-USB communication. Transmission velocities of up to 1 [Mbyte/second] can be achieved. All of the transmission control protocol is contained by the chip. The control signals handle logic signals of 3.3 [V] and 5 [V].

Communication Interface between the PC and the End-User: The software of the Data Acquisition Card was created with Visual Basic 6. The software allows the user to: Start Data Acquisition, End Data Acquisition, change the Sampling Rate, view the temperature of each of the six channels when there are updates according to the Sampling Rate, and store the data which the End-User can view by pressing the View Excel Report.

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Figure 13: Graphical User Interface

Figure 14: Format of Results Displayed with Excel

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5.1.2 Temperature Measurement (Thermocouple)

Type K thermocouples will be used as previously stated in the section of Thermocouples in the chapter of Revision of Bibliography. The thermocouples must be compatible with the rest of the equipment and intended use of the equipment. Therefore, compatibility mostly concerns geometry and chemical resistance of the thermocouple or sheath.

Because the equipment is exposed to reducing atmospheres, it is necessary to place the thermocouples in a protective sheath. In protective sheaths junctions are either grounded, ungrounded or exposed. The junctions placed inside the retort must be ungrounded. They cannot be exposed because the atmospheres would damage the thermocouples and they also cannot be grounded because any possible electric discharge could cause an explosion hazard. Independent of the fact that an ungrounded thermocouple responds the slowest, it is the only possibility.

Initially, there are two thermocouples inside the retort. Both are placed within stainless steel 304 sheaths. One thermocouple is fixed and the other slides axially. The fixed thermocouple serves as a reference and the other scans temperatures axially to discover gradients and response time.

The thermocouples are rated up to 1250 °C, yet the recommended maximum temperature of operation is 900 °C due to the sheath material. When operating with temperatures exceeding 1000 °C, type K thermocouples suffer corrosion which consumes the wire. The higher the temperature and the longer the exposure, the shorter the life of the element. The green scale that forms during exposure should be scraped off with a sharp tool until the bright metal is exposed. The thermocouple should be replace when the wire is less

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IM - 2005 - I - 39 than one-third of its original diameter because the accuracy and reliability are jeopardized.

5.1.3 Adequate Pressure and Flow for Mixing the Processing Gas

The atmosphere within the recipient at a given moment will be a combination of two gases. The approach of selecting a method for creating an atmosphere with known composition percentages will be specifically orientated for the use of hydrogen and nitrogen; although, the selection of a mixing configuration will possibly be extended for future use with other gases if the proper correlations are found.

Once again, the gases used are contained within high pressure industrial bottles. The upper pressure limit of these gases is 2 kPsi. Thus, pressure must be reduced in at least two stages for the gases to be in operating conditions.

For each gas, a separate but repeated configuration as follows can be used. A fixed regulator is used in the first stage. For the second stage, an adjustable regulator is used. The output at the end of the second stage of each gas must be at similar pressures for the mixture to occur.

After the two stage reduction, the possibility of a backflow must be dealt with. A check valve at the end of the second stage guarantees the elimination of the possibility of a backflow from one gas pressure reduction configuration to the other even if there is a slight pressure difference at the outcome of the two second stage reduction.

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Once the pressure is reduced, the flow rate of the each gas must be controlled. The flow rate, which is a volumetric one, permits the proper gas ratio mixture. For example, for a total flow rate of 100 [mL/min] of 80% [N] – 20% [H] into the recipient is accomplished by adjusting the flow of nitrogen to 80 [mL/min] and hydrogen to 20 [mL/min]. So finally, the two flows are previously mixed before entering the recipient. The mixture is simply accomplished by using a “Tee” connection.

The gas mixture must then flow through the recipient. An almost stagnant average flow is desired permitting a thorough filling of the inside of the recipient with the premixed atmosphere. This ultimately, almost complete assures that the surfaces and pores of the green-pieces will be uniformly exposed to the atmosphere.

The previous configuration is used for this project, but an automatic alternative, such as follows, can be a better configuration at the expense of cost.

In spite of pressure fluctuations, the flow within the system must be regulated. The flow state must be guaranteed within a known range of pressures differences. Manual actuation as control of the flow requirement is highly labor intensive and subject to human error. Therefore, a self-adjusting device should be implemented.

The rate of flow through any valve is governed by the pressure drop and size of the opening. For maintaining a constant flow rate, a port must close when the pressure drop increments or open when the pressure drop decreases. And in practice, it is difficult to relate the pressure drop and the size of the opening required for constant flow. The size of the opening is a complicating factor because it must not be regulated by the absolute but by the differential

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IM - 2005 - I - 39 pressure which is subject to the upstream and downstream fluctuations. Thus, the resulting tasks involved are divided into two:

• measurement of the flow • opening or shutting the valve with the information gained.

Matters are complicated further because the correlations found for a particular gas is only valid for gases of similar viscosities. This means that correlations and graphs must be in hand for reference when shooting for a desired flow rate.

Complications are extended some more since the mixture of gases, called a “forming gas”, is adjusted proportionately. In other words, one gas is fixed with respect to the other which is at “set-point”. Consider the following schematic.

Figure 15: Schematic of Configuration for Forming Gas

It is apparent that for the mixing application one flow variable is controlled with reference to another; thus the system is a flow ratio system where the flow of line B is controlled at a pre-set ratio with flow A by the proportional, controlling device. So the control must obtain a measure of the flow in line A. Then, it must transmit the appropriate signal to set the appropriate ratio by adjusting the flow in line B.

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An example should clarify the function of the system.

The system is configured for an atmosphere of 75% gas A and 25% gas B. The user allows a flow of 75 [mL/min] of gas A through valve A. The proportional, controlling device obtains the measured flow. Since it is preset for a 75%-25% atmosphere, it adjusts the flow through valve B to 25 [mL/min]. If the flow of valve A is suddenly decreases to 96 [mL/min], the proportionally, controlling device adjusts the flow through valve B to 32 [mL/min].

Finally, it is important to state explicitly that the system controls the ratio of the gases but not the total quantity of the gases. And if an additional gas is needed for the processing gas, the system could be extended by using either line’s flow signal as a set-point for another flow line C.

The details of the components of the fluid flow system of this project are detailed in the blueprints provided. What follows is an explanation on how to use the fluid flow system configuration.

Below, in the figure: A reducing sintering run, are four steps of the same configuration. The scheme can be divided into a left and right half. The left half is for the inert/purging gas. The right half is for the reducing gas, in this case the hydrogen. For easier interpretation of the flow a color code of green and red is applied. If there is a flow and the components are active, they are colored green. Red represents inactive components.

Oxygen must be removed from the system when commencing the reducing atmosphere of when there is danger of oxidizing the equipment. This is displayed in the first part of the sequence. The second frame corresponds to

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fully operational conditions such as those in a reducing environment with hydrogen. Hydrogen is particularly an essential component of the reducing processing gas8. It is the hydrogen that combines with oxygen from oxides for the reduction of the piece to proceed. It is hydrogen that combines with traces of oxygen in the atmosphere to keep the piece from oxidizing. And when hydrogen does react, hopefully, the only chemical reaction present with oxygen will form water molecules. Then comes the third stage of hydrogen purging. Recall that hydrogen levels must be safely reduced before opening the retort. Finally, the last stage is reserved for the end of the sintering cycle. Once the system is left in this state, the equipment can be safely stored until the next sintering run.

8 There are design considerations of the recipient system due to presence of hydrogen.

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Figure 16: A Reducing Sintering Operation Sequence

5.1.4 Retort construction – Shell

The material of the retort shell should be one that that permits repeatability of experiments. Therefore, the material selected should be unreactive with the atmospheres during the complete sintering process9. An unreactive material keeps corrosion from occurring which could cause premature failure or affect repeatability due to material loss. Additionally, the material selection for the retort considers mechanical properties at elevated temperatures. For

9 The outside of the recipient will be exposed to air at high temperatures. This uncontrolled exposure will certainly oxidize the exterior.

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example, it should have mechanical strength (hot resistance) so that little or no distortion occurs at operating temperatures with the loads. And finally, the material shall permit gas tight atmospheres.

According to any ordinary classification scheme, the materials that are candidates fall into the following scheme:

Metal alloys

Ferrous Nickel Base

Steels

Low alloy

Low-carbon

Plain

High alloy

Stainless

Austenite

Figure 17: Material Candidates for the retort

Suitable candidates available as pipes with nominal diameters between 1.5” and 3” are:

• Stainless steel (302, 304) • Stainless steel (heat resistant 309 or 310) • Mild or plain carbon steel • Inconel 601

Carbon steels and low-alloy steels are the first runners up for elevated temperatures with relatively low costs. Unfortunately, the mayor set back of carbon steels functioning at temperatures above 370 °C is due to the steel

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IM - 2005 - I - 39 decarburizing. The immediate consequence is the loss of mechanical strength of the retort. But the real danger occurs due to hydrogen damage. Ferritic steels have a particular sensitivity to hydrogen. Hydrogen damage associated with low-alloy or carbon steels includes: hydrogen embrittlement, hydrogen attack, and hydrogen blistering. For example, at elevated temperatures hydrogen dissociates and becomes mono-atomic. The hydrogen atom easily penetrates the metal structure and reacts with the carbon to form methane. Methane cannot dissolve in the iron lattice and thus internal gas pressure develops leading to voids, blisters, or cracks. Simpler yet, take for example, the diffusion coefficient for hydrogen in ferritic steel at room temperature is similar to the diffusion coefficient for salt in water. Obviously, all of the previous defects cause catastrophic failure.

The figure: Operating Limits of Various Alloys in a Hydrogen Environment (Nelson Curves) defines the limits of hydrogen in a group of selected alloys. It is apparent that the partial pressures of hydrogen for this particular project is below the stipulated limits (i.e. partial pressures of 0.25 MPA), but the safety margin is extremely narrow, too narrow for any peace of mind. Basically, the concurrent presence of hydrogen and carbon is undesirable; and since the project defines operating conditions with hydrogen, it is concluded that materials with the lowest possible quantities of carbon are required.

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Figure 18: Operating Limits of Various Alloys in a Hydrogen Environment (Nelson Curves) [Ref. 9]

Hydrogen is only one of the operating atmospheres. In fact, it is for the reducing atmosphere inside the retort. The outside of the retort is exposed, at will, to air which at the expected operating temperatures is highly oxidizing. Thus, corrosive mechanisms such as scaling will be present. The figure below will further disregard plain carbon steels and low alloy steels as candidates fit for the retort material. At approximately 675 °C, the rate of loss of material by scaling seems to tend exponentially to infinity. And since the tube thickness will probably be approximately 5 mm., the loss of material is sufficient to consider the tube worthless.

When carbon and low alloy materials provide inadequate corrosion resistance and insufficient high temperature strength, stainless steels are looked upon. In general, stainless steels have at least 11% chromium content; 11% is the amount needed to prevent the formation of rust in unpolluted atmospheres

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IM - 2005 - I - 39 since oxidation resistance is benefited by the chromium content which forms a protective oxide film on the surface.

Figure 19: Effect of temperature on metal loss from scaling for several carbon and alloy steels in air [Ref. 9]

Beyond looking upon stainless steels for their corrosion resistance, they are also selected because of their higher strength over other materials (particularly when compared to low alloy steels) at high temperatures. For this reason, when strength is a concern or when temperatures are so high that any little strength advantage should be considered, austenitic stainless steels are preferred over the other grades of stainless steels (See Figure: General Comparison of the Hot-Strength Amongst Stainless Steels.)

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Figure 20: General Comparison of the Hot-Strength Amongst Stainless Steels [Ref. 10]

The 3xx series, which have the composition of standard austenitic stainless steels, is the suitable series amongst the stainless steels. In general, they possess excellent cryogenic properties, though of no use for this particular application, and good high-temperature strength, a high consideration factor.

The austenitic stainless steel group has a face centered cubic (FCC) structure. Ferritic stainless steals, which have a body-centered-cube crystal structures (BCC), drastically lose their strengths at service temperatures above 640 °C. Therefore, unlike the BCC crystal structures, the face-centered cube crystal structures have the desired high creep strengths. Thus, creep strength is now the mechanical property in play. Nickel addition is responsible for the stabilizing of iron-chromium steels required for creep resistance.

An additional contrast must be made between the consequences of the crystal structures of the steels when interacting with hydrogen. “The body-

59

IM - 2005 - I - 39 centered cubic crystal structure of ferritic iron has relatively small holes between the metal atoms, but the channels between these holes are relatively wide. Consequently, hydrogen has a relatively low solubility in ferritic iron, but a relatively high diffusion coefficient. In contrast the holes in the face-centered cubic austenite lattice are larger, but the channels between them are smaller, so materials such as austenitic stainless steel have higher hydrogen solubility and a lower diffusion coefficient. Consequently, it usually takes very much longer (years rather than days) for austenitic materials to become embrittled by hydrogen diffusing in from the surface than it does for ferritic materials, and austenitic alloys are often regarded as immune from the effects of hydrogen.” [Ref. 20]

And finally, the austenitic stainless steels are the most corrosion resistant compared to ferritic and martensitic stainless steels because of the high chromium contents and also nickel additions.

Corrosion, during any part of the sintering cycle, is always a consideration. It comes in many forms and types but two particular types are pinned. These are knife-line attack and stress-corrosion-cracking. Both of these pertain to inter-granular corrosion.

“Inter-granular corrosion is localized attack along the grain boundaries, or immediately adjacent to grain boundaries, while the bulk of the grains remain largely unaffected. This form of corrosion is usually associated with chemical segregation effects (impurities have a tendency to be enriched at grain boundaries) or specific phases precipitated on the grain boundaries. Such precipitation can produce zones of reduced corrosion resistance in the immediate vicinity.” [Ref. 20]

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The attack is usually related to the segregation of specific elements or the formation of a compound in the boundary. Corrosion then occurs by preferential attack on the grain-boundary phase, or in a zone adjacent to it that has lost an element necessary for adequate corrosion resistance - thus making the grain boundary zone anodic relative to the remainder of the surface. The attack usually progresses along a narrow path along the grain boundary; and, in a severe case of grain-boundary corrosion, entire grains may be dislodged due to complete deterioration of their boundaries. In any case the mechanical properties of the structure will be seriously affected. [Ref. 20]

A classic example is the sensitization of stainless steels or weld decay. Chromium-rich grain boundary precipitates lead to a local depletion of Cr immediately adjacent to these precipitates, leaving these areas vulnerable to corrosive attack in certain electrolytes. Reheating a welded component during multi-pass welding is a common cause of this problem. In austenitic stainless steels, titanium or niobium can react with carbon to form carbides in the heat affected zone (HAZ) causing a specific type of inter-granular corrosion known as knife-line attack. These carbides build up next to the weld bead where they cannot diffuse due to rapid cooling of the weld metal. The problem of knife- line attack can be corrected by reheating the welded metal to allow diffusion to occur. [Ref. 20]10

The other phenomenon of inter-granular corrosion is a consequence of the precipitation of carbide at austenite grain boundaries leaving zones exposed to oxidation. When these chromium depleted zones along the grain boundary unite, they form continuous paths providing the means for propagation of stress corrosion cracking. Stress corrosion cracking (SCC) must be avoided

10 Please note the previous quote is taken “as found” and with extreme difficulty could be said any better.

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IM - 2005 - I - 39 since hydrogen will squeeze through the cracks and create a combustion hazard outside the protective and safe atmosphere.

Remedies for reducing sensitization in austenitic stainless steel are lowering the carbon content (chose a L series such as 304L), heat treat at high temperatures for prolonged times to allow the chromium to re-diffuse into impoverished austenite, or add elements such as titanium which modifies the precipitation process of the carbide (see figure of Family Relationships). If heating is chosen, it is done above 1000°C followed by water quenching to retain the carbon and chromium in solution and so prevent the formation of carbides (see figure of Phase Diagram). Therefore, it is possible to reclaim steel which suffers from carbide precipitation.

But ultimately, sensitization should be combated by minimizing permanency in the sensitizing temperature region. Therefore, the rate of cooling or heating should be as high as possible to reach temperatures above or below the region. Minimization of the transition time through the region is sought and is possible because the carbide precipitation rates are low.

Figure 21: Time-temperature curves showing effect of carbon content on carbide precipitation, which forms in the areas to the right of the various carbon-content curves.[Ref. 6]

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The completion of carbide precipitation is then incomplete and what does precede barely affects the integrity at the grain boundaries. The figure above can be used as a guide to avoid the sensitization range.

In conclusion, temperature-induced micro-structural changes, creep-rupture mechanisms, scaling and oxidation, carburization, and decarburization are critical phenomena that affect the selection and performance of heat-resistant austenitic stainless steels. With all of the previous reference and with the Figure: Family relationships for standard austenitic stainless steels, a selection could be made. But this is selection would be unrealistic. Additional factors like commercial availability and cost also play a vital role in this project. Furthermore, since it is a prototype and “there is always room for improvements”.[Ref. 18], [Ref. 21]

Figure 22: Family relationships for standard austenitic stainless steels

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In summary, 302 and 304 stainless steel may distort since at operating temperatures they lack mechanical strength. Additionally, it is highly possible for corrosion from the break down of surface chrome oxides at the certain operating conditions. 309, 310, and Inconel are all more suitable than 302 and 304. Especially if their cyclic oxidation resistance from the figure is considered. But they are the most expensive when comparing and are only available through importation. So it all this deduction boils down to the use of 304. [Ref. 18], [Ref. 21]

Figure 23: Cyclic Oxidation Resistance [Ref. 10] Cyclic oxidation resistance of a range of superalloys. Thermal cycle was between room temperature and 1000 °C (except for Inconel 601 and 617); 15 min heating, 5 min cooling. For Inconel alloys 601 and 617, cycle was between room temperature and 1095 °C.

Three common sizes are available commercially according to the American Standard Pipe11 and given in the table: Selected American Standard Pipe. A nominal diameter of 3 inches, schedule 40 is readily available and has a larger work volume.

11 A more complete set of pipe sizes are found in the annexed reference folder: Pipes.

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Table 9: Selected American Standard Pipe

Nominal Size, [in.] Outside Diameter, [in.] Wall Thickness, [in.] (Standard No.40) 2 2.375 0.158 2.5 2.875 0.208 3 3.500 0.221

The final recommendation, knowing that a stainless steel 304 tube of a 3 inch nominal diameter has been selected, is to keep below the recommended temperature of the table below and out of the sensitation range as much as possible. Table 10: Maximum service temperatures in dry air, based on scaling resistance [Ref: 6] Grade Intermittent [°C] Continuous [°C] 304 870 925 309 980 1095 310 1035 1150 316 870 925

5.1.5 Flange of the retort

A slip-on flanges is suitable for the retort end that will function as the loading and unloading entrance. The slip-on flange fits over the pipe and is then welded in position. The ease of fitting and welding reduces fabrication costs as well as eliminating the need of ensuring the accuracy of the cut pipe and facilitating the alignment between the pipe and the flange. It is ideal for the low pressure setting inside the retort. And finally, the face of the slip on flange will be of the face-ring-type joint.

Unfortunately, the flange will not conform to standards such as ANSI B 16.5 or BS1560 due to higher and unnecessary costs. Therefore, the operator must keep in mind that the pressure rating of the vessel will not be limited by the seal pressure rating of the o-ring. The operator must consider flange strength and the strength of the tube itself operating at high temperatures.

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The mating piece of the slip-on flange with the face-ring-type joint is a blind flange with the necessary configuration ports for tests. For example, the flange might contain a ¼” NPT female connection for a thermocouple well.

5.1.6 Seal of the Retort

An o-ring seals by blocking any potential leak path between two closely spaced surfaces of a liquid or gas. The o-ring is generally installed in a machined groove in one of the surfaces to be sealed, in this case, the flange on the retort. As the two surfaces are brought together they squeeze the cross section of the o-ring. This squeezing action results in a deformation of the o-ring cross section.

The groove will be made on the removable, blind flange of the retort, the one that functions as a lid, because versatility in the design is desired. For example, if an operator decides a different configuration for interaction within the retort is needed, he or she can simply replace the blind flange without modifying the fixed flange.

For this application a static, axial seal is used. In a static seal application, there is no relative motion between parts of the gland that make contact with the o-ring. Small amounts of movement, such as those caused by thermal expansion, vibration, bolt stretch, or o-ring response to pressure do not alter the consideration of this o-ring as static. The seal is also categorized as axial due to the direction in which the squeeze is applied to the o-ring cross section.

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A variety of materials are available for an o-ring in a static, axial application. The material for the o-ring chosen is nitrile mainly because it is the least expensive and the most readily available. Alternative names for this material are often Nitrite, Buna N, or NBR. Nitrile is a copolymer of butadiene and acrylonitrile and variation in proportions of these polymers is possible to accommodate specific requirements. An increase in acrylonitrile content increases resistance to heat but decreases low temperature flexibility. Unfortunately, the providers of o-rings in Bogotá cannot specify the content. So the assumption must be made that the o-ring in use is a standard nitrile of shore A hardness of 70. Being so, it is functional over a temperature range of approximately -30°C to 120°C. Additionally, the operating temperature over extended times can increase hardness after the o-ring has been heated and cooled.

Compatibility was then checked for. Fortunately, this material is highly compatible with a wide range of environments. Nitrile is recommended by a variety of sources for an environment with either nitrogen or hydrogen. Please check the compatibility chart included in the reference folder of the annexed compact disc for a detailed list of compatibility.

An intrinsic characteristic of an o-ring is its permeability. Permeability is the tendency of gas to pass or diffuse through the elastomer at various rates depending on the elastomer type and nature of the individual compound. This should not be confused with leakage which is the tendency of a fluid to go around the seal. Permeability may be of prime importance in vacuum but is seldom consequential in other applications. It should be understood that permeability increases as temperatures rise, that different gases have different permeability rates, and that the more a seal has greater resistance to permeability the more it is compressed. Generally, harder compounds with more carbon black added have lower diffusion rates. Of the popular

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elastomers, epichlorohydrin and butyl have the lowest permeability, followed by fluorocarbons, polyurethanes, nitrites, heoprenes, polyacrylates, and SBR. Yet, for the majority of applications the rate of gas permeation through the o- ring is inconsequential (a few standard cubic centimeters per year) and standard groove dimensions are applicable12. Please refer to page 75 of the file in the reference folder with the name ORD5700.pdf for empirical values of permeability. Finally, if by chance the composition of the o-ring is known by the commercial supplier in Bogotá and permeability is a concern, the following figure should be used [Ref. 15]:

Figure 24: Effect of Acrylonitrile Content on Permeability of Butadiene-Acrylonitrile Copolymers at 25°C

In low-pressure applications where the confined fluid, such as with this case with gas, exerts little or no pressure on the o-ring, the tendency of the elastomer to maintain its original shape creates the seal. As the O-ring is deformed when the mating surfaces are brought together, it exerts a force against the mating surfaces equal to the force it takes to squeeze it. The areas of contact between the O-ring and the mating surfaces (contact bands) act as a barrier to block the passage of the fluid. From the figure to the right, the tightening force required for any o-ring with a nominal cross section of 1/8 of an inch can be deduced.

12 The approximation formula for leak rate of the seal found on page 70 of Parker’s O-ring Handbook can be used for the uneasy soul doubting the seal quality.

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Figure 25: O-ring compression force [Ref. 2]

The compression, squeeze, created by the force applied is expressed as a percentage of the free-state cross-sectional thickness. In effect, it is:

(O-Ring C/S) - Gland Depth (O-Ring C/S)

For this axial, static flange (face) seal compression should be approximately 20% and may fall anywhere inside the range of 20% to 30%.

Increased o-ring squeeze reduces permeability by increasing the length of the path the gas has to travel (width of ring) and decreasing the area available to the entry of the gas (groove depth). Increasing the squeeze also tends to

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IM - 2005 - I - 39 force the rubber into any small irregularities in the mating metal surface, and thus prevents leakage around the seal.

Thus, when designing a seal, the following issues are usually contemplated: volume change of the o-ring, compression required for type of seal, and tolerances and dimension of the groove.

The volume of an o-ring may change from fluid swell or thermal expansion. It should be known that an o-ring deforms elastically in most designs (an example of a permanent deformation is when an o-ring is used in a crush type seal). And since it deforms elastically, volume is conserved.

Forces generated by fluid swell tend to be slight, whereas forces generated by thermal expansion may be great enough to distort metal parts. Therefore, a gland should provide a minimum of 10% void. That is, the maximum o-ring installed volume should not be more than 90% of minimum gland volume. Calculation based on cross-sectional areas of o-ring and glands are adequate to insure this. This provides ample space to accommodate changes in o-ring volume. Gland fill describes the gland volume vs. o-ring volume. It is simply stated as:

Gland Fill = (o-ring volume) / (gland volume)

Usually about 25% void space or 75% nominal fill suffices the space needed in the groove to allow for volume swell, thermal expansion, and increasing width due to squeeze. Leaving insufficient void space can lead to extrusion of the o-ring into the clearance gap, if there is one, or squeezing the o-ring in two directions, if the seal is a face type seal. A handy reference of Linear Expansion % vs. Volume Swell % is used when there is an expected dimensional change of the o-ring.

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At this moment, please refer to the figure below [Ref. 2]. The fixed o/d ring line indicates the approximate linear change in the cross section of an o-ring when the gland prevents any change in the I.D. with shrinkage, or the O.D., with swell. Hence this curve indicates the change in the effective squeeze on an o-ring due to shrinkage for swell. Note that volumetric change may not be such a disadvantage as it appears at first glance. For example, a volumetric shrinkage of six percent results in only three percent linear shrinkage when the o-ring is confined in a gland. This represents a reduction of only .004" of squeeze on an O-ring having a 0.140" cross-section dimension. The other line indicates linear change in both I.D. and cross-section for a free-state (unconfined) o-ring.

Figure 26: Linear Expansion % vs. Volume Swell %

An example of how the previous information can be used for the retort aids in understanding the importance of verifying that after operation the o-ring does not expand beyond the volume available by the gland. If the o-ring swells it is probably because it has absorbed contents from the environment. The new volume requires a quick calculation of the initial squeeze and the void space available. A possible conclusion can be to replace the o-ring because during

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IM - 2005 - I - 39 higher temperature operating conditions the o-ring will have no space available for expansion. The counterpart possibility is if the o-ring shrinks due to chemical changes at elevated temperatures during prolonged times. If the volume shrinkage is excessive, there is a possibility that the o-ring can no longer create a seal.

Luckily, using the datasheet of a standard nitrile of shore A hardness of 70 fabricated by ERIKS (the sheet is included in the reference folder of seals under the pdf o-ring.info) it can be deduced that no swelling will occur. What does become a problem are the compression set and the linear shrinkage. Please note, compression set and compression are two different concepts. An exertion from the reference information of Parker O-ring Handbook states compression set as follows:

“Compression set is generally determined in air aging and reported as the percent of deflection by which the elastomer fails to recover after a fixed time under specified squeeze and temperature. Zero percent (0%) indicates no relaxation has occurred whereas 100% indicates total relaxation; the seal just contacts mating surfaces but no longer exerts a force against those surfaces.”

It is probably wise to avoid available theoretical and empirical data collected by various o-ring producers because of the many actual variables that differ from the controlled experimental environment. For example, compression set is give in figure 2-12 in the Parker O-ring Handbook at various temperatures, but at a given test time of 70 hours and an o-ring cross-section deformed by 25%. Thus, the figure only gives an idea about the consequences of compression set for the controlled variables of the experiment.13 What should be left from the understanding of compression set is that the dimensions of

13 Even the handbook simply states, “It is easy to go overboard on this property from a theoretical standpoint. Remember that a good balance of all physical properties is usually necessary for optimum seal perform ance. T his is the eternal sealing compromise the seal designer always faces.”

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IM - 2005 - I - 39 the o-ring in place in the flange groove should periodically be checked. Included with the operating manual is a recommended minimum o-ring diameter perpendicular to the sealing surface is given so the operator knows when an o-ring replacement must be made. Please note, even if the minimum diameter of the o-ring is a consequence of another phenomenon besides compression set, it is recommended the replacement be made.

The groove dimensions and design is based upon, yet not limited to it, the previous knowledge. It was decided that a single groove will be placed on the slip on flange. It is a groove designed for a vacuum seal. By creating seals for vacuums, the retort is also suitable for environments without vacuums. The contrary case, where a seal is designed for slight pressure differences, may not be suitable as a vacuum seal. The seal will shoot for eliminating the slightest leakages which are unacceptable in vacuum applications. Therefore, an especially smooth finish in the gland will insure contact between the elastomer and the metal parts, a reduced gland depth will increase the amount of squeeze, and a narrower groove apt for sealing vacuum or gas reduces the exposed o-ring surface where diffusion can occur.

The groove for the o-ring was designed with the following considerations for vacuum and gases: Roughness of groove sides: X = 63 micro inches (1.6 µm Ra) Roughness of all other side: X = 16 micro inches (0.4 µm Ra) Initial compression in excess of 25% is undesirable since it will cause over-compression at higher temperatures.

A Nitrile Rubber (NBR, Acrylonitrile-Butadiene Rubber) o-ring will be used and the selection of the particular o-ring rests highly on the commercially availably. The first consideration was compatibility with the environment. Nitrile, when in contact with either gas, will not experience o-ring swell

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5.1.7 Work load support

Materials used for the work load support must at least be able to handle the cycles of thermal stresses induced when these fixtures are heated and cooled. And once the support is at the holding temperatures, they must retain their mechanical strength during the required holding times. And of utmost importance, the support should not react with either the load or the atmosphere intended for the load.

The incompatibility of the work load support material is checked at the temperature of the work piece and, obviously, with the material of the work piece. Incompatibility is also verified at the pressure inside the recipient.

Aluminum oxide (Al2O3) easily fulfills the requirements of workload support material such as commercial availability and adequate physical, chemical, and mechanical properties. It is a typical ceramic classified as a single oxide; and as such, the nature of its properties is highly statistical. Therefore, the design criteria will aim towards desired properties but can not guarantee them.

As a ceramic, operating temperatures typical of furnaces are handled. Further more, the aluminum oxide will stay inert with its surroundings because of the chemical inertness at temperatures above 1200 °C. But the high temperatures will induce thermal stress. At the high temperatures the support will be in tension. Tension strength of the ceramic is extremely lower than compressive strength14.

14 Properties of aluminum oxide are annexed under Properties of Several Engineering Ceramics.

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A possible solution15 is the reduction of porosity of the support which can be accomplished by altering firing conditions during fabrication. Yet, the benefits of a porous support may outweigh the low strength hazard. A porous support allows interaction amongst the gases of the atmosphere and the pieces16. [Ref. 5]

An alternative “macroscopic solution” is taken. The support will be composed of additive segments. By using segments the total effective length is minimized and so is strain which ultimately translates into a reduction of stress. Stress concentration factors are also considered by opting for rounded geometries.

One of the outstanding properties of the aluminum as the support material for the project is the low thermal expansion coefficient. A flush fit at ambient temperatures of the support within the retort is not a later threat at high temperatures. Many dissimilar materials when in contact and exposed to elevated temperatures cause stress due to the different expansions of the materials. This would not be the case because the thermal expansion coefficient of aluminum oxide (~8.2 um/m-°C) is slightly lower when compared at 800°C with the candidates for the retort material (say for example, stainless steel 304 with ~10.6 um/m-°C).[Ref. 3]

15 Strictly speaking, the magnitude of the potential problem due rupture of the support by thermally induced tension stress is unknown. It is speculated the support will fail. Thus, a prudent design and acknowledgement of the low thermal expansion coefficient should try to minimize the unknown hazardous potential. 16 Gases of the atmosphere will effusion into any void spaces inside the retort because of known behavior to occupy all of the available containing volume.

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Fortunately, the accommodation of the support to the previous constraints is not a complicated issue. Budinski states the following: [Ref. 3]

“Shapes are generated by molding to shape or by machining in the green state and then sintering. The size change and distortion that occur in sintering are accommodated by finish 17 machining with diamond grinding wheels.”

Obviously, shaping can be done manually without the finish machining. The key idea behind the inclusion of the previous statement is that aluminum oxide permits “sanding / grinding” to the desired shape.

Finally, loading and unloading of the pieces onto the support is done outside of the retort. This requires an apparatus to place and remove the support carefully into and out of the retort at the beginning and end of the sintering cycle. The apparatus requires little design. It can simply be a fork like device which slides underneath to lift and lower the support to and from the destination.

5.2 Characterization

Characterization of the whole retort system includes temperature interaction of the retort with the external heating system and flow circulation of the process gas.

5.2.1 Characterization Protocol

The general objective of the characterization is to obtain the response of the equipment which is part of a thermal system and a fluid flow system. The set

17 Page 241 of [Ref. 3]

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IM - 2005 - I - 39 up of this protocol is orientated towards permitting future controlled usage of the equipment. The characterization should allow for automated regulation through use of electronic control systems.

5.2.2 Temperature characterizations

The first step is to characterize the thermocouple within the sheath. In other words, the thermocouple has a time constant and it should be found for an accurate characterization of the equipment. Once this has been done, the characterization of the equipment can proceed.

A transfer function is sought for the equipment where the input is the temperature of the furnace and the output is the inside temperature. By introducing the retort into the furnace at different stable temperatures the step response of the system is obtained. Characterization includes checking the linearity of the system with the results of the stable responses. The transient response looks for the parameter of the time constant which is the time it takes to reach 63.2% of a step change in temperature.

The characterization of the system is performed axially on multiple points of a single axial axis of the retort. And damage of the equipment during this characterization stage is avoided by using nitrogen as the atmosphere inside the retort. It is expected that the time constant increase as the thermocouple slides outward. Finally, as proof that the system is invariant in time, a fixed thermocouple is always logged at the different trials of the thermocouple that displaces itself axially.

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5.2.3 Fluid Flow Characterization

The fluid flow characterization is limited to controlling the rate of volume flow of the gas. Because a measuring needle valve (Metering Swagelock Needle Valve) is used to control flow, the pressure at the inlet of the valve alters the flow quantity. As the valve is opened the flow will increase; and at different inlet pressures the flow will also change. Thus, the flow for three different inlet pressures with nitrogen will be tested to obtaining flow values by opening the valve in a controlled manner. An needle like indicator will extend from the knob. A correlation between flow and number of turns of the knob will be obtained. Once again, this correlation will be found at different pressures.

6 Conclusions

A tubular retort was designed and constructed; yet it still has not been tested. It has not been tested because this equipment is destined to operate in the laboratories of the Universidad de Los Andes and there are a set of rules for installing new equipment. It takes time to go through the proper process.

The equipment designed for sintering is fit for: • Containing gases and in particular, working with hydrogen because it is constructed with austenitic stainless steel 304. • Adjusting low flow rates of gas below 500 mL/minute. • Logging data with its data acquisition card which can then be interpreted with Excel. The equipment still lacks: • A complete characterization.

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7 Suggestions for further development

This project did not achieve 100% of the objective mainly because the system was not characterized; yet the achievements should not be under estimated. If a future student uses this project as a reference, he or she with much less effort, could be familiarized with the subjects concerning the design criteria for proper sintering equipment. Having those solid bases, the current design could continued and improved or even change radically. Thus, the initial objective of this project (design, construction, and characterization) should change and include:

Design, Construct, Characterize, and “Use whatever previous knowledge there is”.

Where are some of the improvements that can be made? Well, the retort system, designed to operate within any type of furnace, is composed of the physical system, fluid flow system, thermal system, and the measurement system.

The physical system, as time will tell, will suffer from corrosion and oxidation. This makes the equipment dangerous when working with hydrogen. Fortunately, this is a prototype created with a low budget and as a prototype it will speak for itself by letting the future designer make further decisions. So when all of the kinks are worked understood and worked out, the investment of more appropriate materials (i.e. SS 310, SS 309, or Inconel 601) can be justified by the long term use of the equipment.

Then there are the improvements of the fluid flow system. The design system heavily relies on creativity and time available because there are many, many

79

IM - 2005 - I - 39 solutions. Only one of the solutions of proper mixing and controlling of the fluid is proposed here after. The solution: fixed, strangled nozzles for mixing. This solution, which is described with more detail in the recollection of information under the reference folder of Mixed Background Information, consists of strangling the flow through nozzles. The main advantage is the constant flow obtained and repeatability of the flow.

The thermal system, which was never really discussed in depth, consisted of the electrical furnace in which the retort was placed. It had a controller with a thermocouple outside of the retort. This obviously affects the possibility of controlling temperature without a large temperature gap between the piece and the retort. Therefore, there are two main suggestions concerning the thermal system. First, design a tubular furnace for the retort. And secondly, the controller of the tubular furnace must a connection of a thermocouple inside the retort.

For the measurement system, which for this project was left to only data acquisition, should be part of the control loop of the thermal system and fluid flow system.

Finally, improvements should also be made in safety. This can be done through alarms. For example, a gas mixer for the furnace atmosphere, based on commercials furnaces described on the Internet, might also include:

• “High Hydrogen Alarm. If the hydrogen level rises above the high alarm value on a gas analyzer, the hydrogen flow will be shut off and a light and horn will be energized. • Low Hydrogen Alarm. If the hydrogen level falls below the low alarm value on the gas analyzer, a light and horn will be energized.

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• Low Nitrogen Pressure. If the inlet nitrogen pressure falls below the minimum necessary for proper gas mixing, the hydrogen flow will be shut off and a light and horn will be energized. • Power Failure. If a power failure should occur, the hydrogen will be shut off and the nitrogen will continue to flow. A light and horn will be energized when the power resumes. If high hydrogen, low nitrogen pressure or the power failure alarms continue for more than approximately 5 seconds, manual reset by the operator is required to restart hydrogen flow.”

There are endless amounts of improvements and there will always be room for more. But at least this project will allow students and others to obtain useful, basic sintering results.

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8 Bibliography

[1] ASKELAND, Donald R., The Science and Engineering of Materials. – 3rd ed., PWS Publishing Company. Boston, 1989

[2] BROWN, Melvin W., Seals and Sealing Handbook – 4th ed., Elsevie Advanced Technology, Oxford, England, 1995.

[3] BUDINSKI, K. G., Budinski, M. K., Engineering Materials: Properties and Selection. – 6th ed., Prentice Hall. New Jersey, 1999

[4] CALLISTER, William D., Materials Science and Engineering: An Introduction. 5th ed., John Wiley & Sons, Inc., 2000

[5] CREASE, A. P., Production Sintering Equipment, ASM Handbook: Powder Metallurgy, Vol. 7 – 5th ed, 1993

[6] DAVIS, Joseph R. , Stainless Steels, ASM International. USA, 1994

[7] DOEBLIN, Ernest O., Measurement systems: application and design. – 4th ed., McGraw-Hill. Singapore, 1990

[8] Evaluation of Hydrogen Embrittlement of SAFKEG 3940A Package in KAMS. Savannah River Technology Center. Publication Date: May 2003

[9] “Elevated-Temperature Properties of Ferritic Steels”, ASM Handbook. Vol. 1. “Properties and selection: irons, steels, and high – performance

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alloys”. ASM International. The Materials Information Society. 5th ed. USA, 1993

[10] “Elevated-Temperature Properties of Stainless Steels”, ASM Handbook. Vol. 1. “Properties and selection: irons, steels, and high – performance alloys”. ASM International. The Materials Information Society. 5th ed. USA, 1993

[11] GERMAN, R. M., Sintering Theory and Practice, John Wiley & Sons, Inc. New York, 1996

[12] “Heat Treating in Vacuum Furnaces and Auxiliary Equipment”, ASM Handbook. Vol. 4 “Heat Treating”. ASM International. The Materials Information Society. 5th ed. USA, 1993

[13] INFORMATION SPECIFIC TO LIQUID HYDROGEN. www.airproducts.com

[14] NAYAR, H. S., “Production Sintering Atmosphere”, ASM Handbook. Vol. 7 “Powder Metallurgy”. 5th ed. USA, 1993

[15] Parker O-Ring Handbook 2001 edition.

[16] Powder Metallurgy Design Manual, – 3rd ed. Metal Powder Industries Federation. 1998

[17] PEASE, Leander., Fundamentals of Powder Metallurgy. Princeton, N.J.: Metal Powder Industries Federation, 2002.

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[18] PYE, D., Practical Nitriding and Ferritic Nitrocarburizing, ASM International, Ohio, 2003

[19] www.barnsteadthermolyne.com

[20] www.corrosion-doctors.org/Forms/intergranular.htm

[21] www.engineeringtoolbox.com

[22] www.omega.com

[23] www.scottsemiconductor.com/pures/hydrogen.html

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TABLE OF CONTENTS

1 INTRODUCTION...... ¡ERROR! MARCADOR NO DEFINIDO. 2 OBJECTIVE...... ¡ERROR! MARCADOR NO DEFINIDO. 3 REVISION OF BIBLIOGRAPHY...... ¡ERROR! MARCADOR NO DEFINIDO. 3.1 OVERVIEW OF POWDER METALLURGY...... ¡ERROR! MARCADOR NO DEFINIDO. 3.2 OVERVIEW OF SINTERING...... ¡ERROR! MARCADOR NO DEFINIDO. 3.2.1 Physical Phenomenon - Diffusion...... ¡Error! Marcador no definido. 3.2.2 Schematic of creating a sintering cycle...... ¡Error! Marcador no definido. 3.2.2.1 Delubing stage...... ¡Error! Marcador no definido. 3.2.2.2 P reheating stage / Presintering...... ¡Error! Marcador no definido. 3.2.2.3 Hot Stage...... ¡Error! Marcador no definido. 3.2.2.4 Cooling Stage...... ¡Error! Marcador no definido. 3.2.2.5 Slow cooling stage...... ¡Error! Marcador no definido. 3.2.2.6 P urging, a used as needed stage...... ¡Error! Marcador no definido. 3.2.3 Parameters ...... ¡Error! Marcador no definido. 3.2.3.1 Temperature – Heat...... ¡Error! Marcador no definido. 3.2.3.2 Gases...... ¡Error! Marcador no definido. 3.2.3.2.1 Classification of Gases...... ¡Error! Marcador no definido. 3.2.3.2.2 Combustion (Explosion) of gases...... ¡Error! Marcador no definido. 3.2.3.2.3 Ignition (Auto-ignition) of gases...... ¡Error! Marcador no definido. 3.2.3.2.4 Gases for the atmosphere...... ¡Error! Marcador no definido. 3.2.4 Sintering Equipment...... ¡Error! Marcador no definido. 3.2.4.1 Batch Furnaces...... ¡Error! Marcador no definido. 3.2.4.2 Continuous Furnaces...... ¡Error! Marcador no definido. 3.2.4.3 Heating Units...... ¡Error! Marcador no definido. 3.2.4.4 Gas Pressure and Flow Equipment...... ¡Error! Marcador no definido. 3.2.4.5 Additional Comparisons...... ¡Error! Marcador no definido. 3.2.4.6 Thermocouples...... ¡Error! Marcador no definido. 4 METHODOLOGY...... ¡ERROR! MARCADOR NO DEFINIDO. 5 RESULTS AND ANALYSIS...... ¡ERROR! MARCADOR NO DEFINIDO.

5.1 DETAILS OF CREATING THE RETORT...... ¡ERROR! MARCADOR NO DEFINIDO. 5.1.1 Six Channel Data Acquisition...... ¡Error! Marcador no definido. 5.1.1.1 General Architecture of the System...... ¡Error! Marcador no definido. 5.1.1.2 Block Architecture of Te mperature Sensors...... ¡Error! Marcador no definido. 5.1.1.3 Implementation...... ¡Error! Marcador no definido. 5.1.2 Temperature Measurement (Thermocouple) ...... ¡Error! Marcador no definido. 5.1.3 Adequate Pressure and Flow for Mixing the Processing Gas...... ¡Error! Marcador no definido. 5.1.4 Retort construction – Shell...... ¡Error! Marcador no definido. 5.1.5 Flange of the retort...... ¡Error! Marcador no definido. 5.1.6 Seal of the Retort...... ¡Error! Marcador no definido. 5.1.7 Work load support...... ¡Error! Marcador no definido. 5.2 CHARACTERIZATION...... ¡ERROR! MARCADOR NO DEFINIDO. 5.2.1 Characterization Protocol...... ¡Error! Marcador no definido. 5.2.2 Temperature characterizations ...... ¡Error! Marcador no definido. 5.2.3 Fluid Flow Characterization...... ¡Error! Marcador no definido. 6 CONCLUSIONS...... ¡ERROR! MARCADOR NO DEFINIDO. 7 SUGGESTIONS FOR FURTHER DEVELOPMENT...... ¡ERROR! MARCADOR NO DEFINIDO. 8 BIBLIOGRAPHY...... ¡ERROR! MARCADOR NO DEFINIDO.

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Listing of Figures

Figure 1: The steps in diffusion bonding [Ref. 1]...... ¡Error! Marcador no definido. Figure 2: Diffusion of atoms to points of contact [Ref. 1]...... ¡Error! Marcador no definido. Figure 3: Generalization of activation energy...... ¡Error! Marcador no definido. Figure 4: Schematic of an Arbitrary Sintering Cycle .¡Error! Marcador no definido. Figure 5: Stress-strain curves for 304 stainless steel at vary in hydrogen concentration (given in wt%) [Ref. 8] ...... ¡Error! Marcador no definido. Figure 6: Furnace Equipment [Ref. 19] ...... ¡Error! Marcador no definido. Figure 8: Tubular Recipient Development Model...... ¡Error! Marcador no definido. Figure 9: Prototype...... ¡Error! Marcador no definido. Figure 10: Data Acquisition Card ...... ¡Error! Marcador no definido. Figure 11: General Architecture of System ...... ¡Error! Marcador no definido. Figure 12: Block Architecture of a Temperature Sensor ...... ¡Error! Marcador no definido. Figure 13: Graphical User Interface...... ¡Error! Marcador no definido. Figure 14: Format of Results Displayed with Excel ...¡Error! Marcador no definido. Figure 15: Schematic of Configuration for Forming Gas ...... ¡Error! Marcador no definido. Figure 16: A Reducing Sintering Operation Sequence ...... ¡Error! Marcador no definido. Figure 17: Material Candidates for the retort...... ¡Error! Marcador no definido. Figure 18: Operating Limits of Various Alloys in a Hydrogen Environment (Nelson Curves) [Ref. 9]...... ¡Error! Marcador no definido. Figure 19: Effect of temperature on metal loss from scaling for several carbon and alloy steels in air [Ref. 9] ...... ¡Error! Marcador no definido. Figure 20: General Comparison of the Hot-Strength Amongst Stainless Steels [Ref. 10] ...... ¡Error! Marcador no definido. Figure 21: Time-temperature curves showing effect of carbon content on carbide precipitation, which forms in the areas to the right of the various carbon-content curves.[Ref. 6]...... ¡Error! Marcador no definido. Figure 22: Family relationships for standard austenitic stainless steels ...... ¡Error! Marcador no definido. Figure 23: Cyclic Oxidation Resistance [Ref. 10] ...... ¡Error! Marcador no definido. Figure 24: Effect of Acrylonitrile Content on Permeability of Butadiene- Acrylonitrile Copolymers at 25°C...... ¡Error! Marcador no definido. Figure 25: O-ring compression force [Ref. 2]...... ¡Error! Marcador no definido. Figure 26: Linear Expansion % vs. Volume Swell %...... ¡Error! Marcador no definido.

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Listing of Tables

Table 1: Conforming Methods for Green Pieces.¡Error! Marcador no definido. Table 2: Classification of Some Common Gases¡Error! Marcador no definido. Table 3: Limits of Hydrogen by Volume Percentage...... ¡Error! Marcador no definido. Table 4: Gas Cylinder Specifications ...... ¡Error! Marcador no definido. Table 5: Types of Batch Furnaces...... ¡Error! Marcador no definido. Table 6: Types of Continuous Furnaces...... ¡Error! Marcador no definido. Table 7: Accuracy of Variable Area Flowmeters.¡Error! Marcador no definido. Table 9: Selected American Standard Pipe...... ¡Error! Marcador no definido. Table 10: Maximum service temperatures in dry air, based on scaling resistance [Ref: 6]...... ¡Error! Marcador no definido.

87 Appendix 1: Thermocouples Table: Summary of Thermocouples [Ref. 7] Laws of Thermocouple Behavior Implications and application of the Thermocouple Laws 1 The thermal emf of a thermocouple with The lead wires connecting the two junctions junctions at T1 and T2 is totally unaffected may be safely exposed to an unknown by temperature elsewhere in the circuit if the and/or a v arying temperature environment two metals used are each homogeneous. without affecting the voltage produced.

Schematic 1 of

2 Laws 2 and 3 make it possible to insert a If a third homogeneous metal C is inserted v oltage-measuring device into the circuit into either A or B, as long as the two new actually to measure the emf,. The metal C thermo-junctions are at like temperatures, represents the internal circuit between the the net emf of the circuit is unchanged instrument binding posts. The instrument irrespective of the temperature of C away can be connected in any of the two from the junctions. schematics to the right.

Schematic 2 of 3 If metal C is inserted between A and B at one of the junctions, the temperature of C at any point away from the AC and BC The thermocouple junctions may be junctions is immaterial. As long as the soldered or brazed, thereby introducing a junctions AC and BC are both at the third metal, without affecting the readings. temperature T1, the net emf is the same as if C were not there.

Schematic 3 of

4 All possible pairs of metals need not be calibrated since the individual metals can If the thermal emf of metals A and C is E AC each be paired with one standard (platinum and that of metals B and C is ECB, then the thermal emf of metals A and B is E + E . is used) and calibrated. Any other AC CB combinations then can be calculated; calibration is not necessary.

Schematic 4 of

5 In using a thermocouple to measure an unknown temperature, the temperature of one of the thermojunctions (called the If a thermocouple produces emf E then its 1 ref erence junction) must be known by some junctions are at T and T , and E then at T 1 2 2 2 independent means. A v oltage and T , then it will produce E + E when the 3 1 2 measurement then allows us to get the junctions are T and T . 1 3 temperature of the other (measuring) junction from calibration tables. This law also implies linearity.

Schematic 5 of

Appendix 2: Specific Gravity Heptanes 3.459 of Gases [21] Hexane 2.973 Specific Hy drogen 0.069 Hy drogen gravity is the 1.268 ratio between chloride - HCl Hy drogen sulfide the density 1.190 Spec if ic Grav ity - H2S (mass per 1) Isobutane 2.01 unit volume) -SG- of the actual Isopentane 2.48 gas and the Kry pton 2.89 density of Methane - CH4 0.554 air.Gas Methy l Chloride 1.74 Acety lene Natural Gas 0.907 0.60 - 0.70 (ethyne) – C2H2 (ty pical) Ai r 1) 1.000 Neon 0.696

Ammonia - NH3 0.596 Nitric oxide – 1.037 NO Argon – A 1.379 Nitrogen - N 0.967 Arsine 2.69 2 Nitrous oxide - Blast Furnace 1.530 1.02 N O gas 2 Nonane 4.428 Butadiene 1.869 Octane 3.944 Butane - C4H10 2.067 Oxy gen - O 1.105 Carbon dioxide 2 1.529 – CO2 Pentane 2.487 Carbon Phosgene 1.39 0.967 monoxide - CO Propane - C3H8 1.562 Carbureted 0.63 Propene Water Gas (Propy lene) - 1.451 Chlorine - Cl2 2.486 C3H6 Coke Oven Gas 0.44 Sasol 0.42 Cyclobutane 1.938 Silane 1.11 Cyclopentane 2.422 Sulf ur Dioxide - 2.264 SO Cyclopropane 1.451 2 Toluene- Decane 4.915 3.176 Methy lbenzene Digestive Gas Water Gas (Sewage or 0.8 0.71 Biogas) (bituminous) Xenon 4.53 Ethane - C2H6 1.049 1) Ethylene NTP- Normal Temperature and Pressure - 0.975 is defined as air at 20oC (293.15 K, 68°F) (Ethene) – C2H4 and 1 atm (101.325 kN/m2, 101.325 kPa, Fluorine 1.31 14.7 psia, 0 psig, 30 in Hg, 760 torr) Helium - He 0.138

Appendix 3: Gases - Explosive and Flammability Concentration Limits [21]

The lower and upper explosion concentration limits for some common gases may be found in the table below. Some of the gases are common as fuels.

“Lower Explosive or “Upper Explosive or Flammable Limit” Flammable Limit” Fuel Gas (LEL/LFL) (UEL/UFL) (%) (%) Acety lene 2.2 100 Ammonia 15 28 Arsine 5.1 78 Butane 1.8 8.4 Carbon Monoxide 12 75 Cyclopropane 2.4 10.4 Ethane 3 12.4 Ethylene 2.7 36 Ethyl Chloride 3.8 15.4 Hy drogen 4 75 Isobutane 1.8 9.6 Methane 5 15 Methy l Chloride 10.7 17.4 Propane 2.1 9.5 Propy lene 2.0 11.1 Silane 1.5 98

Note! The limits are for air at 20ºC and atmospheric pressure.

Appendix 4: Fuels and Ignition Temperatures

Ignition temperatures for some common fuels as butane, coke, hydrogen, petroleum and more

The ignition temperature - the minimum temperature required to ignite a gas or vapor in air without a spark or flame being present - for some common fuels can be found below:

Temperature Fuel o ( C) Acety lene 305 Benzene 415 Bituminous coal 300 Butane 420 Carbon 700 Carbon monoxide 300 Coal-tar oil 580 Coke 700 Ethane 515 Heavy hydrocarbons 750 Hy drogen 500 Light gas 600 Light hydrocarbons 650 Methane 580 Naphtha 550 Natural gas 600 Peat 227 Petroleum 400 Producer gas 750 Propane 480 Semi anthracite coal 400 Wood 300

Appendix 5: Properties of Selected Tubing

CORRECTION FACTOR TEMP Multiply Value found on appropriate table above by correction factor below to determine Working Pressure adjusted for temperature. °F °C Aluminum Copper Steel 304SS 316SS Alloy 400 200 93 1.00 0.80 0.95 1.00 1.00 0.88 400 204 0.40 0.50 0.86 0.93 0.96 0.76 600 316 0.77 0.82 0.85 0.76 800 427 0.58 0.76 0.79 0.76 1000 538 0.69 0.76 1200 649 0.30 0.37

Table: Stainless Steel Tubing Suggested Working Pressure (KSI) Tube O.D. Tube Wall Thickness [thousandths of an inch] [in.] 0 12 14 16 20 28 35 49 65 83 95 109 120 134 156 188 1/16 5.6 6.8 8.1 9.4 12.0 1/8 8.5 10.9 1/5 5.4 7.0 10.2 1/4 4.0 5.1 7.5 10.2 1/3 4.0 5.8 8.0 3/8 3.3 4.8 6.5 1/2 3.5 4.7 6.2 5/8 4.0 5.2 6.0 3/4 3.3 4.2 4.9 5.8 7/8 3.6 4.2 4.8 1 3.1 3.6 4.2 4.7 1 1/4 3.3 3.6 4.1 4.9 1 1/2 3.4 4.0 4.9 2 3.6

Values based on ASTM A-269 high quality fully annealed ty pe 304 or 316 seamless Stainless Steel Hy draulic tubing. NOTE: For welded and drawn tubing a degrading factor must be applied for weld integrity: For Double Welded Tubing multiply suggested pressure rating by 0.85. For Single Welded Tubing multiply suggested pressure rating by 0.80.

The following table contains the commercial tubing of interest and available for immediate delivery in Bogotá:

Table: Commercial Tubing in Bogotá Size Material Thickness Inside Diameter Price [$/mt.] 1/8 SS 316 0.035” 0.055 10500 ¼ SS 316 0.035” 0.180 11500 ¼ SS 316 0.049” 0.152 12300 ¼ SS 316 0.065” 0.120 16800

Appendix 6: Blueprints of Sintering Equipment